Journal of Molecular Structure 919 (2009) 146–153
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Molecular structure and electrical properties of some phosphonates, phosphine-oxides and phosphates O.V. Prezhdo a, B. Gawdzik b, V.V. Zubkova b, V.V. Prezhdo b,* a b
Department of Chemistry, University of Washington, Seattle, WA 98195-1700, USA Institute of Chemistry, Jan Kochanowski University, Che˛cinska Str. 5, 25-020 Kielce, Poland
a r t i c l e
i n f o
Article history: Received 2 March 2008 Received in revised form 3 August 2008 Accepted 26 August 2008 Available online 11 September 2008 Keywords: Dipole moment Molar Kerr constant Conformational properties Phosphonates Phosphine-oxides Phosphates
a b s t r a c t Dipole moments and molar Kerr constants of several phosphonates, phosphine-oxides and phosphates were measured in benzene solution and calculated both quantum-chemically and by the dipole and tensor-additive scheme. The data indicate that the compounds exist in thermodynamical equilibrium of several conformers. A single experimental or theoretical approach cannot independently identify the prevailing molecular structures. Only a combined analysis of the three types of data on the experimental and calculated dipole moments and molar Kerr constants allowed us to fully characterize the conformational properties of the phosphonate, phosphine-oxide and phosphate compounds under investigation. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Organic phosphates, phosphonates and phosphine-oxides play many important roles in nature, science and technology. Thus, organic phosphates and phosphonates take part in the development of living matter, while phosphonates and phosphine-oxides find wide technological applications. For example, phosphoryl-tethered beta-cyclodextrins were synthesized, and the thermodynamic parameters of the complexes between the hosts and chiral guests were studied [1]. Computational and experimental investigations of the second-order nonlinear optical properties of diphenylphosphine oxide and substituted arylphosphine oxides were reported in Refs. [2,3]. Arylphosphine oxides with donor substituents (OH, NH2) in position 4 form intermolecular hydrogen bonds of the P@O HAO type in the solid state [3] and in solution, as is the case with tributylphosphinoxide [4]. The propensity of phosphates to intermolecular interactions draws wide interest. The reactions of molecular complex formation between uridine on one side and adenosine, cytidine, thymidine, adenosine 50 -monophosphate or cytidine 50 -monophosphate on the other side were studied in Ref. [5]. Similarly to posphines, both intermolecular hydrogen bonding and universal interactions define the properties of molecular aggregates containing phosphates, for example, in adjacent
* Corresponding author. Tel.: +48 41 349 7025; fax: +48 41 349 7062. E-mail address:
[email protected] (V.V. Prezhdo). 0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.08.026
phospholipid molecules [6,7]. Many biological molecules strongly interact with water. Hydrogen bonding of water molecules with the structural analogues of the nucleic acid’s phosphate group was studied in Ref. [8]. The role of the H1 helix in the ETS domain was investigated in Ref. [9]. The results demonstrated the importance of the dipole-facilitated phosphate contact in the DNA binding. The properties of the natural mononucleotides were analyzed based on their infrared spectra in aqueous solution [10]. The mean square dipole moments of the 1:1 complexes of tri-n-butyl phosphate (TBP) with 1-hexanol, 1-heptanol and 1-octanol in benzene were determined in Ref. [11]. The mechanism of dipolar complexation involving TBP and these three long-chain aliphatic alcohols was investigated with a modified Plait method using dielectric measurements in a non-polar medium (benzene) at frequency 455 kHz and T = 303.16 K [12]. The 1:1-adducts (C5H5)3Ln TEP of tricyclopentadienides containing trivalent lanthanides (Ln) with triethylphosphate (TEP) were synthesized, isolated and investigated [13]. The dielectric constants and dipole moments of the individual molecules and adducts were measured, and the charge molecular distributions were discussed. Multiple molecular conformations determine the structure and properties of many systems involving organic phosphates, phosphonates and phosphine-oxides. Thus, in Ref. [14] alkyl diisopropyl phosphates were metallated by sBuLi/TMEDA at 78 C at the alkyl and isopropyl groups in a ratio, which was strongly influenced by the steric effects. The influence of the flexibility and electrostatic interactions of the sugar ring on the DNA sugar–phosphate back-
O.V. Prezhdo et al. / Journal of Molecular Structure 919 (2009) 146–153
bone conformation and the mechanism of the conformational DNA transition was investigated in Ref. [15]. Conformations of molecules containing several rotationally active groups bound to the same atom are particularly interesting from the point of view of the influence of inter- and intramolecular interactions on molecular structure [16]. Phosphates, phosphonates and phosphine-oxides belong just among such compounds. The stereochemistry of trialkylphosphates is actively discussed in the literature [17–29], and the structure of the trimethylphosphate (TMP) molecule has been the focus of especially many studies [17–22]. The analysis of the vibrational spectra of trimethyl-, triethyland triphenylphosphates [20] testifies to the presence of two rotational isomers with (approximate) C3 and C3V symmetries. The normal modes and frequencies for several conformations of the (CH3O)3PO and (CH3O)3PS molecules were investigated in Ref. [21]. The TMP conformations were studied using dipole moment and electro-optical Kerr effect measurements in Ref. [22]. The infrared (IR) and Raman spectra of TMP were recorded in the vapor, liquid and crystalline states as well as in solvents of varying polarity [19]. Restricted rotations around the three PAO bonds allow TMP to exist in many conformations with C3v, C3 and C1 symmetries. The ab initio calculations predicted that the three O@PAOAC dihedral angles are equal to 44.8° in the C3 symmetry conformer and 34.7°, 50.3° and 178.2° in the C1 symmetry conformer. The energy difference between the two conformations DH = DHC3 DHC1 was calculated to be 4.2 kJ mol1. The ab initio molecular orbital calculations on the TMP molecule were done using 6-31G* and 6-31G** basis sets, both at the RHF and MP2 levels of theory [18]. Three minima corresponding to the C3, C1, and Cs symmetries in the increasing energy order were located. At the MP2/6-31G** level, the energy difference between the C3 and C1 conformers was 0.56 kcal/mol, while that between the C3 and Cs conformers was 1.43 kcal/mol. The barrier for the conformer interconversion, C1 , C3, was also computed at the HF/6-31G** and MP2/6-31G** levels. At the MP2/6-31G** level the C1 ) C3 barrier was found to be 2.20 kcal/mol, while that for C3 ) C1 was 2.76 kcal/mol [18]. Conformational isomerism of TMP was studied de novo in Ref. [17]. TMP monomers were isolated in low temperature xenon matrices at different substrate temperatures and were characterized using the FTIR spectroscopy. At the lowest temperature, three different conformers with overall molecular symmetries C3, C1 and Cs were shown to co-exist in the matrix. Increase of the sample temperature led to conformational cooling, with the two minor conformers successively depopulated. Initially, the less stable Cs conformer converted to the C1 form and, at higher temperatures, the C1 conformer converted to the global energy minimum, C3 [17]. The dipole moments of compounds (RO)3PO (R = CH3, C2H5, i-C3H7, t-C4H9) were investigated earlier in Refs. [23,24]. The authors explained the observed reduction of the experimental dipole moments in this series either by changes in the electronic density around the phosphorus atom or by changes in the relative populations of the rotational isomers. The conclusion about the existence of a mixture of (CH3O)3PO conformers received in Ref. [20] was confirmed in Ref. [25]. Participation of the isomers with the cis–gauche–gauche or gauche–gauche–gauche arrangements of the OAC and P@O bonds and the trans-orientation of one alkoxygroup were deduced on the basis of a joint analysis of the IR data and dipole moments for TMP and TEP, and additionally electrooptical Kerr effect for TMP [26–28]. The electronographic study of TMP [29] determined the existence of a mixture of two C3 symmetry conformers in the 1:3 ratio favoring the trans–trans–trans conformer. Motivated by the wide spectrum of applications and the earlier studies we investigate the stereochemistry of a number of new
147
phosphates, phosphonates and phosphine-oxides. In particular, we focus on the stereochemical roles of the hydrocarbon chain branching and the OH group participation the intramolecular hydrogen bond (IHB). The phosphates, phosphonates and phosphine-oxides under investigation were selected based on their agricultural uses [30–32]. The synthesis procedure together with the 1H NMR and IR spectra used to identify these compounds can be found in the above references. Presently, we study the electric properties of the synthesized compounds. 2. Experimental The dipole moments of the compounds under investigation were measured in benzene at 25 °C using the SH-2-11 apparatus produced by the Experimental Design Office of Automation, Angarsk, Russia [33]. The SH-2-11 apparatus allows us to obtain the dielectric constant and density of solutions to the fourth place accuracy. The dielectric permittivity (e) and density (q) were measured for the dilute solutions of the studied compounds with concentrations of up to 103 mol/l. The values of the molar polarizations (P21) of the dissolved substances extrapolated to infinitive dilution were calculated according to Hedestrand [34]. The derivatives of the dielectric constant (aH = de/dN2) and density (bH = dq/dN2) taken with respect to the solute molar fraction (N2) are given in Table 1. The table also presents the molar polarizations (P21), the deformation polarizations (PD = 1.05RD, where RD is molar refraction), and the measured (lexp) and calculated (lcalc) dipole moments of the compounds. The Kerr constants (B) of the compounds under study were measured in solutions according to Ref. [35]. The refractive indices (nD) of the solutions were measured using the IRF-23 refractometer. The mathematical analysis of the experimental data was performed according to Ref. [36] in order to obtain the molar Kerr constants (mK). The concentration dependence of the measured characteristics of the solutions were expressed as e = e1(1 + aLx2), q = q1(1 + bLx2), n = n1(1 + cx2), B = B1(1 + dx2), where x2 is the mass fraction of the solute and e1, q1, n1 and B1 are the corresponding characteristics of the solvent. Extrapolation to infinite dilution was carried out. The values of aL, bL, c, d, and mK are given in Table 2. The quantum-chemical calculations were carried out with the Gaussian 98 suite of programs [37]. Density functional theory with the B3LYP functional and the 6-31g* atomic orbital basis was employed for this purpose. The solvent effects were represented using the polarized continuum model with the Gaussian keyword ‘‘POLAR”. The molecular geometries were fully optimized to the default accuracy. The calculated values of the dipole moments were taken directly from the program outputs. The molar Kerr constants were calculated from the values of the dipole moments and polarizability tensors according to Ref. [38]. 3. Results and discussion Molecules such as (C6H5)3P@O and their derivatives with other bulky substituents Alk(C6H5)2P@O cannot be planar due to the steric effects [39]. Our analysis of the dipole moment data using the Stuart–Briegleb models as well as preliminary geometry optimizations with molecular mechanics leads to the conclusion that the investigated phosphineoxides (compounds 1–6 in Table 1) exist in a ‘‘propeller” conformation. The propeller is formed by the two aryl rings and the alkyl group. The propeller ‘‘blades” are at 40.5–42.5° angles with the plane of the CAP@O molecular fragment [40]. The molecular conformation of compound 5 is shown in Fig. 1. The molecular dipole moments of compounds 1–6 (Table 1) were calculated with the help of the tensor-additive scheme [41] using the group and bond dipole moments and molecular geometries. The following bond and group dipole moments (DM) were used: DM(C sp3 C sp2 ) = 0.67 D; DM(COOEt) = 1.80 D, h° = 89°; DM (OH) = 1.60 D, h° = 63° [42];
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Table 1 Experimental dipole moments (lexp) measured in benzene at 25 °C and dipole moments calculated using the vector addition scheme (lcalc) for some phosphonate, phosphate and phosphine-oxide molecules Compound code
Compound formula
aH
bH
P21, (cm3)
15.80 ± 0.12
0.504 ± 0.002
603.53 ± 4.22
17.74 ± 0.04
0.521 ± 0,002
15.04 ± 0.03
PD, (cm3)
lexp (D)
lcalc (D)
83.83
5.01 ± 0.04
4.96
689.31 ± 1.59
88.71
5.42 ± 0.01
4.86
0.494 ± 0,001
604.15 ± 1.26
93.59
5.00 ± 0.01
4.71
17.33 ± 0.02
0.858 ± 0.019
676.47 ± 0.70
98.48
5.32 ± 0.01
4.83
14.90 ± 0.10
0.714 ± 0.004
607.74 ± 3.43
103.60
4.97 ± 0.03
4.65
11.60 ± 0.21
0.891 ± 0.002
496.53 ± 7.23
108.48
4.36 ± 0.08
4.14
9.65 ± 0.09
0.316 ± 0.001
360.08 ± 3.09
59.97
3.80 ± 0.03
3.60
10.17 ± 0.06
0.755 ± 0.001
405.95 ± 1.97
64.85
4.09 ± 0.02
3.94
8.92 ± 0.10
0.680 ± 0.001
369.86 ± 3.59
69.73
3.84 ± 0.04
3.84
8.36 ± 0.07
0.494 ± 0.002
360.93 ± 2.31
74.62
3.74 ± 0.03
3.37
10.38 ± 0.04
0.564 ± 0.001
436.50 ± 1.47
79.73
4.18 ± 0.02
3.86
7.95 ± 0.04
0.638 ± 0.003
357.69 ± 1.55
84.62
3.66 ± 0.11
3.29
10.80 ± 0.06 12.04 ± 0.07 5.53 ± 0.01 8.99 ± 0.03
0.527 ± 0.004 0.454 ± 0.002 0.518 ± 0.004 0.432 ± 0.003
218.47 ± 1.07 226.51 ± 0.97 142.11 ± 0.99 182.79 ± 1.55
29.50 35.63 30.15 35.88
3.04 ± 0.04 3.25 ± 0.03 2.35 ± 0.03 3.09 ± 0.02
– – 2.34 –
OH
O 1
Ph 2 P
O
OH
2
Ph P 2 O
3 Ph P 2
OH
O
4
Ph
2
P
OH
O 5
Ph P 2 O
CO Et 2
Ph P 2
CO 2 Et
6
O
OH
7
( EtO )2 P
O
OH
8
( EtO ) P 2 O 9
( EtO ) P 2
OH
O 10
( EtO ) P 2
OH
O 11
( EtO ) P 2
CO 2 Et
O 12
( EtO) 13 14 15 16
2
P
CO 2 Et
(n-C4H9O)3PO (n-C5H11O)3PO (t-C4H9O)3PO (i-C5H11O)3PO
Shown are the derivatives of the dielectric permittivity (aH = de/dN2) and density (bH = dq/dN2) of the solutions with respect to the solute molar fraction (N2), as well as molar polarizations (P21) and deformation polarizations (PD).
Table 2 The experimental values of the specific (sK2) and molar (mKexp) Kerr constants of the investigated phosphine-oxides and phosphonates with intramolecular H-bond, measured in benzene at 25 °C Compound code
aL
bL
c
d
(sK2) 1014
mKexp 1012
1 2 7 8
0.0491 0.0458 0.0488 0.0342
0.00613 0.00467 0.00221 0.00127
0.143 0.0021 0.00036 0.0025
1.854 0.508 2.425 0.3508
13.39 7.263 2.548 9.949
38.33 21.79 5.66 23.55
The parameters aL, bL, c and d provide the concentration dependence of the solutions dielectric permittivity, density, refractive index and Kerr constant, as described in the text.
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Fig. 1. Molecular conformation of compound 1 in the optimized geometry.
DM(OEt) = 1.04 D, h° = 72° [38]; DM(P@O) = 2.95 D; DM(PAO) = 0.60 D, DM(PAC6H5) = 1.09 D [40]; DM[(C6H5)3P@O] = 4.59 D [43]. The molecular dipole moments were computed using the Gilman formula [44]:
l 2 ¼ l20 þ þ2
n X
i¼1 n X
l2i þ 2
n X
li ðl0x aix þ l0y aiy þ loz aiz Þ cos hi
i¼1
li lj ðaix ajx þ aiy ajy þ aiz ajz Þ cos hi cos hj
i–j
Here, l0 is the dipole moment of the fixed polar C3P@O fragment calculated on the basis of the above data; l0[, l0y, l0z are prol0 on the chosen coordination axes; li, lj are the group jections of ~ dipole moments of n freely rotating polar groups (OH, CH3, CO2Et); hi and hj are the angles formed by the vectors of the group dipoles with the rotation axes ai, aj; aix, aiy, aiz are the unit vectors along the i rotation axis in the chosen system of coordinates. Dipole moments of compounds 1–6 are larger than the dipole moments of compounds 7–12 due to the presence of the bulky phenyls with hindered rotation in the former set. The dipole moments of compounds 3–6 are somewhat smaller than those of compounds 1–2, whose OH groups possess less spatial freedom and can form IHBs of the P@O HAO type. The existence of the IHBs is supported by the analysis of the H1 NMR spectra obtained during identification of these compounds [30–32]. Compounds 7 and 8 also form the IHBs of the same type. This issue will be discussed further below. Considering the internal rotations relative to the PAO axis in compounds 7–12 one can assume a threefold rotation barrier for each ethoxy group, which are generated by the C2H5 radicals of the two ether groups and the phosphoryl bond. An example of the conformation of molecule 7 is given in Fig. 2. Intramolecular interactions with the P@O group, which has one of the largest bond dipole moment of 2.95 D, can lead to energetic instabilities of the
conformations involving the P@O and OC2H5 groups. For this reason Ref. [26] discussed the formal possibility of 16 configurations for phosphates. In contrast, intramolecular interactions cannot create a significant potential barrier between the cis- and gauche-orientations of the OC2H5 groups in relation to the phosphoryl group. The cis-/gauche-conformational freedom together with steric interactions among the three groups lead us to consider a variety of additional conformations, for example, cis–cis–gauche or gauche– gauche–gauche. One can expect a simpler set of conformations in compounds 13–16 with three identical substituent chains forming dihedral angles somewhere in the range between 120° and 240°. Considering trialkylphosphates 13–16 we begin with compound 15, which has the bulkiest substituents. Ref. [27] showed that the IR spectra of this compound are similar in melt and solutions, independent of the solvent polarity. This result indicates that a single rotational isomer of compound 15 is stabilized in solution. The IR spectra of compounds 13 and 14 testify to a thermodynamic equilibrium of several conformers [27]. Comparison of the spectra of compounds 13–16 with the spectra of trimethyl- and triethylphosphates, measured in identical conditions, shows that transition from the less bulky CH3 group to the bulkier substituents is accompanied by decreasing high-frequency component of the (P@O) doublet. The high-frequency component corresponding to the more polar isomer is absent altogether in the IR spectrum of compound 16. The above analysis of trialkylphosphates leads to the six conformations shown in Fig. 3. The dipole moment of compound 15 strictly corresponds to conformation (a) with u1 = u2 = u3 = 120°. Compound 16 presents an equilibrium of two conformers. The larger dipole moment of compound 16 relative to compound 15 and the characteristic dependence of the (P@O) intensity on the environment permittivity [27] indicates that a second, more polar conformer is present in the thermodynamic equilibrium. Since the (P@O) frequency remains unchanged on the environment permittivity, structure (a) is the prevailing form of compound 16 [27]. In such case, the relative conformer concentrations determined from the IR spectra [27] and the experimental dipole values of compound 16 can be used in order to estimate the dipole moment of the second conformer. The l2 value obtained according to the equation
ðlexp Þ2 ¼ ðl1 Þ2 x þ ðl2 Þ2 ð1 xÞ; where lexp = 3.09 D and l1 = lgauche–gauche–gauche = 2.40 D, completely agrees with the dipole moment calculated the form (b) with u1 = 0; u2 = u3 = 120°. The dipole moment values and the IR spectra of compounds 13 and 14 [27] support conformational equilibrium of structures (a) and (b). Compounds 13–16 differ in the length and branching of the hydrocarbon chains. It is known [29] that ethers of acids
O
O a) R
P O O
O
R
O
O O R
Fig. 2. Molecular conformation of compound 7 in the optimized geometry.
P O O
R R
R
P
O O
R R
R O
R
O
O O
R R
O
e)
P O O R R
c)
P
R O
d)
O
b)
O R
f) P O O R R
Fig. 3. Possible conformation of trialkylphosphates characterized by the following ranges of the bond angles: (a) 120° (u1, u2, u3) 180°; (b) u1 = 0, 120° (u2, u3) 180°; (c) u1 = 0, 120°u2180°; 180° u3 240°; (d) u1 = 0, 180° u2 240°; 120° u3 180°; (e) u1 = u2 = 0, 120° u3 180°; (f) u1 = u2 = u3 = 0.
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containing the P atom are characterized by the tetrahedral geometry, which is partly distorted at the P@O group: the O@PAO angle is larger than the OAPAO angle. trans conformations of the alkyl chains create extra spatial congestion in comparison with the gauche conformations. In cases of large branched radicals, as in Fig. 3, one can expect that the main intramolecular interaction occurs by steric repulsion. The steric repulsion should destabilize the trans-orientation relative to gauche and create a notable potential barrier in the cis-orientation. An additional steric repulsion is present, for instance, when the three groups CH3 interact with each of the C(CH3)3 groups. All these factors stabilize the gauche– gauche–gauche form in the most sterically congested compound 15. With less bulky substitutes, thermodynamic equilibrium can allow each of the forms (b)–(f) as a minor partner. Note, however, that interaction of even one pair of CH3 groups in conformation (e) can greatly increase the system energy. Ref. [45] estimated the steric repulsion of the CH3 groups in the trans-position. The large value of the repulsion energy, which is equal to ECH3 CH3 ¼ 17 kcal/ mol at rC. . .C = 2.65 Å excludes from consideration structure (e), and even more so structure (f). Conformation (c) with rC. . .C = 2.95 Å is disfavored by ECH3 CH3 ¼ 5:3 kcal/mol. The available experimental material does not allow one to choose between forms (b) and (c). One can argue that form (b) is more likely to be observed in equilibrium with form (a), because both (a) and (b) have the same arrangement of the two alkyl groups. The data given in Ref. [46] lead one to believe that the dihedral angle involving the OAPAO chain changes considerably depending on the degree of branching of the hydrocarbon radical in dialkylphosphites. Steric effects in both dialkyl- and trialkylphosphates displace the conformation equilibrium. The resulting conformational heterogeneity of the compounds under investigation can influence their chemical activity [27]. Compounds1, 2, 7 and 8 deserve particular attention, since they can form IHBs between the phosphoryl and hydroxyl groups as in P@O HAO. The study of the polarizability and first hyperpolarizability tensors for the IHB molecules shows that the molecular electrical properties play an important role in the creation and stabilization of IHB [47]. The molar Kerr constant is especially sensitive to the formation of hydrogen bonds [48]. The experimental values of the molar Kerr constant (mKexp) measured in dilute benzene solutions at T = 298 K are presented in Table 2. The corresponding calculated values (mKcalc) obtained using the B3LYP
density functional theory with the 6–31g* atomic orbital basis and the polarized continuum model of the solvent are shown in Table 3. The molecular geometries were fully optimized. Table 3 also contains the calculated values of the molecular dipole moments. Both the dipole moments and, even more so, the molar Kerr constants are sensitive to the solvent polarity [49]. Therefore, including the solvent effects in the calculation was essential. The Mulliken atomic charges localized on the key molecular fragment in compounds 1, 2, 7 and 8 are shown in Table 4. The data of Table 4 indicates that the IHB changes the electronic density distribution. The largest changes in the atomic charges (Dq) are seen with the atoms directly involved in the IHB. The absolute value of the charge localized on the oxygen atom of the P@O group increases the most (DqO5 ¼ 0:042 0:05). The change of the charge localized on the phosphorous varies within the range DqP = 0.018–0.036. The smallest changes are seen with the charge of the hydrogen atom involved in the O H–O bridge (DqH = 0.0003–0.0054), even though the corresponding value for the intermolecular H-bond in the water dimer (H2O)2 is significantly larger (DqH = 0.011) [50]. The solvent favors H-bond polarization and, therefore, creates an additional small change in the atomic charges. Significant amounts of energy can be gained by the O HAO hydrogen bonding. For example, DEexp = 20.934 kJ/mol in the water dimer (H2O)2. In the case of a strong H-bond as in (H5O2)+, the energy benefit is enormous (DEcalc = 132.278 kJ/mol) [51]. The stabilizing effect of IHBs on the long-lived rotamer of EE-2,6-di-[2(furan-2-yl)vinyl]pyridine is discussed in Ref. [52]. For the compounds under consideration, the calculations predict modest energy gains. For the P@O HAO IHBs, seen in the A type structures defined in Table 4, the energy gains increase in the following compound sequence 7, 1, 2, 8 and are equal to 8.942, 8.950, 10.594, 12.215 kJ/mol, respectively. Compounds 7 and 8 can form additional IHBs of the PAO(ether) HAO type (structure C in Table 4). The corresponding energy gains are 10.515 and 13.805 kJ/mol. In all cases the IHBs stabilize the molecules. The both lengths shown in Table 5 and the bond angles shown in Table 6 are slightly modified as a result of the IHB formation. In all cases the length of the OAH bond in the O HAO bridge increases (DlOAH = 0.002–0.011 Å. The change is less significant with the PAO(ether) HAO bridge compared to the P@O HAO bridge. The corresponding P@O bonds grow by DlP@O = 0.006–0.009 Å and the PAO(ether) bonds elongate by 0.004–0.011 Å. If the IHB creates
Table 3 Calculated dipole moments (lcalc), polarizability ellipsoids (bii), and molar Kerr constants (mKcalc) of compounds 1, 2, 7 and 8 Compound codea
Theoretical methodb
lx (D)
ly (D)
lz (D)
lcalc (D)
bxx (Å)
byy (Å)
bzz (Å)
bxy (Å)
bxz (Å)
byz (Å)
mKcalc 1012 esu
1A
I C6H6 I C6H6 I C6H6 I C6H6 I C6H6 I C6H6 I C6H6 I C6H6 I C6H6 I C6H6
1.294 1.371 2.042 2.259 1.220 1.298 1.215 1.380 2.026 2.184 1.624 1.797 2.131 2.177 1.506 1.625 1.775 1.960 2.056 2.254
3.069 3.361 1.135 1.152 3.634 3.905 0.113 0.100 2.287 2.445 0.387 0.431 0.141 0.220 1.926 2.075 0.592 0.632 1.305 1.411
3.246 3.500 2.064 2.941 3.794 4.110 3.109 3.435 4.329 4.596 2.331 2.490 2.163 2.254 4.753 5.090 0.036 0.041 2.800 2.974
5.264 5.757 4.006 4.584 5.393 5.816 3.340 3.703 5.298 5.645 2.867 3.101 3.040 3.211 5.344 5.731 1.871 2.060 3.711 3.990
34.779 38.301 31.148 34.292 36.057 39.983 31.608 35.179 22.318 24.394 23.369 25.827 23.828 24.300 23.973 26.424 25.516 28.190 25.265 27.534
26.100 28.058 30.734 33.798 27.211 29.412 33.000 36.890 18.398 19.784 18.260 19.707 17.827 19.722 19.833 21.476 18.819 20.234 20.131 21.542
26.584 28.560 26.800 29.102 28.719 31.102 28.967 31.898 15.86 16.886 15.472 16.496 15.616 16.842 17.662 18.925 17.627 18.862 16.806 17.772
3.889 4.594 1.381 1.659 4.312 5.151 1.895 2.366 0.989 1.160 0.681 0.806 1.444 1.552 1.074 1.278 0.506 0.600 0.680 0.796
0.364 0.444 1.788 2.122 0.252 0.305 2.502 3.075 1.050 1.229 0.275 0.324 0.200 0.221 1.138 1.353 0.423 0.502 1.058 1.218
1.008 1.151 1.787 2.119 0.078 0.083 1.359 1.695 0.270 0.309 0.743 0.851 0.556 0.307 0.227 0.264 0.156 0.180 0.930 1.051
136.1 186.1 19.7 34.4 166.0 230.9 34.89 32.57 125.8 165.4 36.46 21.67 107.2 223.7 189.2 92.9 254.1 367.6 195.9 298.5
1B 2A 2B 7A 7B 7C 8A 8B 8C a b
A indicates IHB of the P@O HAO type, B indicates no IHB, and C indicates IHB of the PAO(ether) HAO type, as shown schematically in Table 4. I implies isolated molecule; C6H6 implies molecule in benzene represented by the continuum model.
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O.V. Prezhdo et al. / Journal of Molecular Structure 919 (2009) 146–153 Table 4 Mulliken atomic charges for molecular fragments in compounds 1, 2, 7 and 8 Atoma
Methodb
P
I C6H6 I C6H6 I C6H6 I C6H6 I C6H6 I C6H6 I C6H6 I C6H6
C1 C2 O3 H4 O5 O6 O7
Mulliken charge 1A
1B
2A
2B
7A
7B
7C
8A
8B
8C
0.9053 0.9050 0.5374 0.5309 0.0867 0.0862 0.6448 0.6476 0.1568 0.1535 0.6497 0.6525
0.8692 0.8687 0.5236 0.5282 0.0994 0.0992 0.6250 0.6243 0.1549 0.1526 0.6001 0.6020
0.9062 0.9059 0.5287 0.5282 0.0948 0.0945 0.6494 0.6523 0.1505 0.1477 0.6473 0.6502
0.8756 0.8747 0.4995 0.4995 0.1036 0.1042 0.6189 0.6186 0.1515 0.1524 0.6024 0.6050
1.2119 1.2119 0.5311 0.5307 0.0796 0.0788 0.6399 0.6439 0.1589 0.1549 0.6084 0.6124 0.5674 0.5660 0.5248 0.5259
1.1823 1.1803 0.5267 0.5272 0.0945 0.0943 0.6228 0.6220 0.1568 0.1534 0.5662 0.5686 0.5687 0.5677 0.5305 0.5316
1.1530 1.2119 0.5147 0.5308 0.0654 0.0788 0.6263 0.6437 0.1569 0.1551 0.5577 0.6123 0.5619 0.5660 0.5482 0.5259
1.2108 1.2115 0.5273 0.5269 0.0863 0.0858 0.6442 0.6483 0.1527 0.1495 0.6077 0.6119 0.5660 0.5645 0.5244 0.5257
1.1722 1.1715 0.5027 0.5028 0.0948 0.0966 0.6193 0.6191 0.1527 0.1543 0.5642 0.5673 0.5661 0.5650 0.5233 0.5243
1.1559 1.1565 0.5215 0.5214 0.0902 0.0901 0.6333 0.6335 0.1495 0.1486 0.5848 0.5851 0.5489 0.5487 0.5552 0.5557
a The atoms are numbered according to the following schemes. A indicates IHB of the P@O HAO type, B indicates no IHB, and C indicates IHB of the PAO(ether) HAO type
O5
O5
H4
P C1
A b
H
H
P
O3
H
H O 7 O C1 6 H4 C2
C2 H
O3 H
C
I implies isolated molecule; C6H6 implies molecule dissolved in benzene as represented by the continuum model.
Table 5 Calculated bond lengths (Å) in the molecular fragments shown in Table 4 for compounds 1, 2, 7 and 8 Bond
Compound code 1A
1B
2A
2B
7A
7B
7C
8A
8B
8C
PAC1 P@O5 C1AC2 C2AO3 O3AH4 PAO6
1.847 1.515 1.558 1.420 0.980 –
1.849 1.506 1.549 1.427 0.972 –
1.847 1.515 1.559 1.419 0.981 –
1.849 1.508 1.537 1.430 0.970 –
1.824 1.491 1.558 1.419 0.978 1.620
1.823 1.485 1.549 1.425 0.971 1.626
1.825 1.484 1.551 1.435 0.973 1.630
1.825 1.491 1.558 1.421 0.978 1.620
1.829 1.485 1.537 1.431 0.970 1.619
1.823 1.489 1.549 1.427 0.973 1.630
Table 6 Calculated bond angles (°) in the molecular fragments shown in Table 4 for compounds 1, 2, 7 and 8 Angle
C1APAO5 PAC1AC2 C1AC2AO3 C2AO3AH4 C1APAO6 C1APAO7 PAO6AC
Compound code 1A
1B
2A
2B
7A
7B
7C
8A
8B
8C
112.36 112.13 110.33 106.20 – – –
114.90 116.02 109.63 106.59 – – –
112.56 111.95 110.39 106.24 – – –
112.81 112.58 106.16 107.32 – – –
114.43 112.43 110.42 106.63 102.83 107.21 122.42
117.14 115.16 109.61 106.66 100.44 106.12 122.89
116.99 115.33 110.92 107.73 100.25 106.74 122.64
114.55 112.19 110.43 106.59 102.92 107.15 122.42
114.99 113.25 105.98 107.42 98.70 110.32 122.86
117.59 116.69 110.49 106.51 104.35 101.02 119.00
six-membered quasiaromatic [51], the changes in the atomic charges and bond lengths can be much more significant [53]. The bond angles involving the atoms directly participating in the IHB formation in the compounds under investigate change within 0.25–2.7° for the C1APAO2 angle and 5.35–9.2° for the C1APAO7 angle. The solid state NMR analysis of the molecular structure showed that compound 1 was synthesized as a racemic mixture [54]. In crystal, the molecules of compound 1 are connected in pairs by two intermolecular H-bonds. Compounds 2, 7 and 8 should have similar crystal structures. Dissolving these compounds in benzene breaks the intermolecular H-bonds and allows formation of the
IHBs, resulting in a dynamic equilibrium involving the IHB structures A and C, and structure B without IHB, as defined in Table 4. It is worth mentioning here that a weak IHB within the C5 peptide structure can influence the ability of the NH group to participate in the IHB of the NAH O type [55]. Which structure dominates can only be determined by a combination of several methods, as discussed below. Fig. 4 compares the dipole moments and molar Kerr constants for compounds 7 and 8. In both cases the experimental points are situated on the lines connecting the theoretical points corresponding to the A and B conformers. This fact suggests that conformer C does not participate in the equilibrium. Then, the
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O.V. Prezhdo et al. / Journal of Molecular Structure 919 (2009) 146–153
mK.10
A number of conformers exist in the thermodynamic equilibrium involving trialkylphosphates in solutions. Tri(t-butyl)phosphate is an exception, whose unique conformation is determined by the strong steric effects. Conformers with cis- or gauche-positioning of the alkoxy groups relative to the P@O bond dominate the equilibrium. The gauche–gauche–gauche conformer is stabilized in tri(t-butyl)phosphate. Increasing of the size of the substituent disfavors the trans-orientation of the alkoxy group. Phosphonates and phosphine-oxides containing OH groups, which are capable of intramolecular hydrogen bonding with the P@O group, exist in a thermodynamic equilibrium involving two forms, with and without the P@O HAO hydrogen bond. This fact was established using a combination of experimental and theoretical data for the molecules dissolved in non-polar solvents such as benzene.
12
esu
800 C’
0
B
B’ E
E’
C
-800
A’
-1600 A
0
20
40 μ2,D2 References
Fig. 4. Graphical comparison of squared dipole moments (l2, D2) and molar Kerr constants (mK 1012 esu) of conformations A, B and C of compound 7 and A0 , B0 and C0 of compound 8; E and E0 are the experimental points, accordingly. The A, B and C conformations are defined in Table 4.
Fig. 5. Equilibrium between conformations A with intramolecular H-bond and B without intramolecular H-bond for compound 1.
lengths of the AE and BE legs for compound 7, and A0 E0 and B0 E0 for compound 8 become directly related to the weights of the A and B conformers in the thermodynamic equilibrium. Conformer B dominates the equilibrium for both compounds 7 and 8. Conformation C is not available in compounds 1 and 2. The equilibrium between A and B is illustrated in Fig. 5. This fact simplifies the analysis of the equilibrium weights. For instance, taking the calculated dipole moments and molar Kerr constants from Table 3 and the experimental values from Table 2, we obtain the following systems of equations for compound 1:
5:7572 x þ 4:5842 y ¼ 5:512 186:1x þ 34:4y ¼ 38:33; where ‘‘x” and ‘‘y” designate the weights of conformers A and B, respectively. The first equation is based on the dipole moments, and the second equation is based on the molar Kerr constants. The solution gives: x = 0.33; y = 0.67, indicating that conformer B is more important in the thermodynamic equilibrium than conformer A. This property of compound 1 is similar to that of compounds 7 and 8. 4. Conclusions The results of our studies indicate that the molecules of phosphonates, phosphine-oxides and phosphates are characterized by significant conformational heterogeneity, which depends on the structure, size and electrical properties of the substituents bonding with the P@O group.
[1] Y. Liu, L. Li, X.L. Li, H.Y. Zhang, T. Wada, Y. Inoue, J. Org. Chem. 68 (2003) 3646. [2] K. Chaneching, M. Lequan, R.M. Lequan, C. Ranser, M. Brazouskas, A. Fort, J. Mater. Chem. 5 (1995) 649. [3] K.L. Kott, C.M. Whitaker, R.C. McMahon, Chem. Mater. 7 (1995) 426. [4] G. Zundel, Adv. Chem. Phys. 111 (2000) 145. [5] L. Lomozik, R. Jastrzab, J. Sol. Chem. 33 (2004) 1243. [6] F. Lairion, E.A. Disalvo, Langmuir 20 (2004) 9151. [7] M.A. Villarreal, S.B. Diaz, E.A. Disalvo, G.G. Montich, Langmuir 20 (2004) 7844. [8] P. Kolandaivel, R. Kanakaraju, Int. J. Mol. Sci. 4 (2003) 486. [9] H. Wang, L.P. McIntosh, B.J. Graves, J. Biol. Chem. 277 (2002) 2225. [10] B. Hernandez, J.M. Gavira-Vallejo, R. Navarro, A. Hernanz, J. Mol. Struct. 565 (2001) 259. [11] S.K. Dash, Phys. Chem. Fluids 38 (2000) 49. [12] S.K. Dash, B.B. Swain, J. Phys. Soc. Jpn. 65 (1996) 3366. [13] B. Kanellakopulos, R. Vonammon, C. Apostolidis, E. Dornberger, R. Maier, J. Muller, X. Zhang, J. Organomet. Chem. 483 (1994) 193. [14] F. Hammerschmidt, S. Schmidt, Eur. J. Org. Chem. IS 12 (2000) 2239. [15] V.I. Pechenaya, Mol. Biol. 96 (1992) 934. [16] A.N. Vereshchagin, V.E. Kataev, A.A. Bredikhin, A.P. Timosheva, G.I. Kovylyaeva, E.Kh. Kazakova, The Conformational Analysis of Hydrocarbons and Their Derivatives, Nauka, Moscow, 1990 (in Russian). [17] I. Reva, A. Simao, R. Fausto, Chem. Phys. Lett. 406 (2005) 126. [18] L. George, K.S. Viswanathan, S. Singh, J. Phys. Chem. A 101 (1997) 2459. [19] V. Sablinskas, A. Horn, P. Klaeboe, J. Mol. Struct. 349 (1995) 157. [20] F.S. Mortimer, Spectrochim. Acta 9 (1957) 270. [21] L.S. Mayants, E.M. Popov, M.I. Kabachnik, Optika i Spektroskopiya 7 (1959) 170. [22] M.J. Aronej, L.H.L. Chia, R.J.W. Le Fewre, J.D. Saxbay, J. Chem. Soc. 7 (1964) 2948. [23] W. Waclawek, W. Guzowski, Roczn. Chem. 42 (1968) 2183. [24] Yu.V. Kolodyazhnyi, V.G. Tkalenko, A.P. Sadimenko, N.A. Kardanov, O.A. Osipov, Zh. Obshch, Khimii 45 (1975) 749. [25] F. Herail, CR Acad. Sci. 261 (1965) 3375; J. Chim. Phys. Phys. Chim. Boil. 68 (1971) 274. [26] O.A. Raevskii, A.N. Vereshchagin, F.G. Halitov, Izv. AN SSSR, ser. Khim. (1972) 353. [27] O.A. Raevskii, A.N. Vereshchagin, Yu.A. Donskaya, A.G. Abulkhanov, Ya.A. Levin, Izv. AN SSSR, ser. Khim. (1976) 2013. [28] V.I. Katolichenko, Yu.P. Egorov, Yu.Ya. Borovikov, G.A. Golik, Zh. Obshch, Izv. AN SSSR, ser. Khim. 43 (1973) 2490. [29] H. Oberhammer, Z. Naturforsch. 28 (1973) 1140. [30] B. Gawdzik, R. Obara, J. Zon´, C. Wawrzen´czyk, Phosphorus Sulfur Silicon 117 (1996) 139. [31] B. Gawdzik, M. Szczepaniak, J. Nawrot, A. Pra˛dzin´ska, B. Gabrys´, K. Dancewicz, C. Wawrzen´czyk, in: H. Górecki, Z. Dobrzan´ski, P, Kafarski (Eds.), Chemistry for Agriculture, New Agrochemicals and Their Safe Use for Health and Environment, vol. 5, Czech-Pol Trade, Prague–Brussels, 2004, p. 119. [32] B. Gawdzik, M. Szczepaniak, A. Pra˛dzin´ska, J. Nawrot, B. Gabrys´, K. Dancewicz, C. Wawrzen´czyk, in: H. Górecki, Z. Dobrzan´ski, P, Kafarski (Eds.), Chemistry for Agriculture, Development in Production and Use of New Agrochemicals, vol. 2, Czech-Pol Trade, Prague–Brussels, 2005, p. 162. [33] Adnopcroe cdblenekmcndo CCCP, Ecnpoqcndo lkz bpvepeybz lb'kernpbxecroq gpoybwaevocnb, lb'kernpbnxecrb[ gonepm b gkoneocnb ;blrocnb» No. 1026532, rk. Co/4/04, 1983. [34] G. Hedestrand, Z. Phys. Chem. B 32 (1929) 428. [35] O.V. Prezhdo, L. Switek, V.V. Zubkova, V.V. Prezhdo, Acta Phys. Pol. 108 (2005) 429. [36] C.G.L. Le Fewre, R.J.W. Le Fewre, J. Chem. Soc. (1953) 4041. [37] M.J. Frish, et al., Gaussian 98, Gaussian 98, Pittsburg, PA, 1998. [38] A.N. Vereschagin, Polarizability of Molecules, Nauka, Moscow, 1980 (in Russian). [39] V.I. Minkin, O.A. Osipov, YU.A. Zhdanov, Dipole Moments in Organic Chemistry, Plenum Press, New York, 1970.
O.V. Prezhdo et al. / Journal of Molecular Structure 919 (2009) 146–153 [40] A.N. Timosheva, G.V. Romanov, S.G. Vulfson, A.N. Vereshchagin, T.Ya. Stepanova, A.N. Pudovik, Izv. AN SSSR, ser. Khim. (1982) 604. [41] E.A. Ishmaeva, A.N. Vereshchagin, N.G. Husainova, Z.A. Bredikhina, A.N. Pudovik, Izv. AN SSSR, ser. Khim. (1982) 604. [42] D.D. Tanner, T.S. Gilman, J. Am. Chem. Soc. 85 (1963) 2892. [43] A.L. Mc Clellan, The Table of Dipole Moments, W.H. Freeman & Co., V.1, 1963; V.2, 1989. [44] T.S. Gilman, J. Am. Chem. Soc. 88 (1966) 1861. [45] E.L. Eliel, N.L. Allinger, S.J. Angyal, G.A. Morrison, Conformational Analysis, Interscience Publishers, New York, 1965. [46] O.A. Raevsky, Ju. Donskaya, F.C. Khalitov, E.I. Vorkunova, A. Levin, Phosphorus 5 (1975) 241. [47] Z. Latajka, G. Gajewski, A.J. Barnes, H. Ratajczak, J. Mol. Struct. 844–845 (2007) 340.
153
[48] O.V. Prezhdo, V.V. Prezhdo, J. Mol. Struct. 526 (2000) 115. [49] V.V. Prezhdo, G.V. Tarasova, O.V. Prezhdo, S.A. Tyurin, N.I. Ivanov, T.N. Kurskaya, Acta Phys. Pol. A 88 (1995) 419. 89 (1996) 47. [50] P. Schuster, in: A. Pullman (Ed.), From Diatomics to Biological Macromolecules, Willey-Interscience, NY, 1977, pp. 123–128. [51] V.P. Bulychev, N.D. Sokolov, in: N.D. Sokolov, Hydrogen Bond, Nauka, Moscow, 1981, pp. 10–29. [52] A. Spalletti, G. Cruciani, U. Mazzucato, J. Mol. Struct. 612 (2002) 339. [53] N.S. Golubev, G.S. Denisov, V.M. Shraiber, in: N.D. Sokolov, Hydrogen Bond, Nauka, Moscow, 1981, pp. 212–254. [54] J. Gajda, A. Jeziorna, W. Ciesielski, W.M. Potrzebowski, W.W. Prezdo, M.J. Potrzebowski, Solid State Nucl. Magn. Reson. 31 (2007) 153. [55] S. Scheiner, T. Kar, J. Mol. Struct. 844–845 (2007) 166.
