Journal of Molecular Spectroscopy 236 (2006) 189–200 www.elsevier.com/locate/jms
High-resolution infrared and theoretical study of gaseous 1,2,5-selenadiazole in the 600–1400 cm 1 range F. Hegelund b
a,*
, R. Wugt Larsen b, R.A. Aitken c, M.H. Palmer
d
a Department of Chemistry, University of Aarhus, DK-8000 Aarhus C, Denmark Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen OE, Denmark c School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9ST, Scotland, UK d School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, Scotland, UK
Received 19 December 2005; in revised form 25 January 2006 Available online 3 March 2006
Abstract The Fourier transform gas-phase IR spectrum of natural isotopic 1,2,5-selenadiazole, C2H2N2Se, has been recorded with a resolution of ca. 0.0025 cm 1 in the wavenumber region 600–1400 cm 1. The three a-type bands, m2 (A1), m4 (A1), m5 (A1), the two b-type bands m11 (B1), m12 (B1), and the c-type band m14 (B2) for each of the isotopologues C2H2N280Se and C2H2N278Se have been analyzed using the Watson model. Ground state rotational and quartic centrifugal distortion constants as well as upper state spectroscopic constants have been obtained from the fits. The rotational constants, harmonic and anharmonic frequencies, and vibration–rotation constants (alphas, aA;B;C ) have been predicted by quantum chemical calculations using a cc-pVTZ basis at the MP2 and B3LYP methodology levels, m and compared with the present experimental data. Although the rotation constants are marginally closer to experiment from the MP2 calculations, in general the B3LYP frequencies and alphas are closer to experiment. 2006 Elsevier Inc. All rights reserved. Keywords: 1,2,5-Selenadiazole; Infrared spectrum; Rotational constants; Vibrational frequencies; Harmonic frequencies; Anharmonic frequencies; Equilibrium structure
1. Introduction The gaseous five-membered ring heterocycles are attractive for high resolution infrared studies since the rotational structure of their fundamental bands is well resolved in spectra obtained from contemporary infrared Fourier transform spectrometers. In addition, the size of these molecules makes them well suited for theoretical studies of molecular properties, and the main purpose of the present study is to obtain precise spectroscopic parameters, and to compare with results from ab initio calculations for a molecule of this type. In a previous paper we investigated 1,2,5thiadiazole [1]. The present investigation is devoted to the analogous selenium compound 1,2,5-selenadiazole.
*
Corresponding author. Fax: +45 8619 6199. E-mail address:
[email protected] (F. Hegelund).
0022-2852/$ - see front matter 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2006.01.011
C2H2N2Se is a conjugated five-membered heterocycle having selenium directly connected to two nitrogen atoms:
In their microwave study from 1967 Blackman et. al. [2] showed that 1,2,5-selenadiazole is a planar C2v molecule with the a-inertial axis coinciding with the twofold rotation axis. The vibrational assignment of the fundamental bands was performed in 1968 by Benedetti and Bertini [3] from infrared and Raman spectra, and this assignment was confirmed in 1995 by El-Azhary [4] from force field
190
F. Hegelund et al. / Journal of Molecular Spectroscopy 236 (2006) 189–200
calculations at MP2 level of theory. More recently, El-Azhary and Al-Kahtani calculated improved vibrational frequencies at the B3LYP, MP2, and HF level [5]. To our knowledge no other spectroscopic studies of 1,2,5-selenadiazole have been published, and thus we initiated a high-resolution infrared study of this molecule. In the present paper we investigate six fundamental bands of 1,2,5-selenadiazole in the 600–1400 cm 1 region namely: three a-type bands m5 (A1) at 726.3 cm 1, m4 (A1) at 1005.2 cm 1, m2 (A1) at 1359.8 cm 1; two b-type bands m12 (B1) at 880.3 cm 1, m11 (B1) at 1234.1 cm 1; and the c-type m14 (B2) at 833.8 cm 1. Together with the published ground state microwave measurements from [2] this yields data for determination of precise ground state constants for the molecule. Most of the bands studied are unperturbed or only slightly affected by resonance, and upper state constants have been obtained for the bands from the Watson model. The sample used for recording the spectra contains a mixture of Se isotopologues in natural abundance, and we have assigned and analyzed six fundamental bands for both of the two most abundant isotopologues C2H2N280Se (49.8%) and C2H2N278Se (23.5%). For both molecules rotational constants, alphas, and vibrational band centers are predicted at the MP2 and B3LYP levels using a cc-pVTZ basis and compared with experimental values. 2. Experimental 1,2,5-Selenadiazole was prepared [6] by addition of 1,2diaminoethane (in DMF) to selenium dioxide (in DMF) at the temperature of 120 C. The product was a colourless liquid, with 1H NMR shift (dH, CDCl3) of 9.35 ppm [literature (dH, CCl4) 9.24 ppm]. Further details are available in [7]. The high-resolution mid-infrared gas-phase absorption spectrum of 1,2,5-selenadiazole vapor was recorded at room temperature on the Bruker IFS 120 HR Fourier transform spectrometer located at the H.C. Ørsted Institute at the University of Copenhagen. The instrument settings are listed in Table 1. The synthesized sample of 1,2,5-selenadiazole was transferred to a White type multipass absorption cell with a base length of 16 cm via a clean Pyrex glass/PTFE gas-handling system. A sample pressure of 1.0 Torr was monitored using a calibrated 10-Torr capacitance manometer. Table 1 Instrument settings Spectral range (cm 1) Instrument resolution (cm 1) FTS aperture (mm) Detector Beam splitter Number of scans Path length (cm) Sample pressure (Torr)
550–1400 0.00250 1.50 Broadband MCT Ge on KBr 312 184 1.0
The absolute wavenumber scale of the transmittance spectrum was calibrated against observed lines of H2O reported by Guelachvili [8], and scaled according to Brown and Toth [9]. Line positions from the absorbance spectrum were generated using the peak-finding function in the ORIGIN 7.0 software package (Microcal software). The precision of the line positions is estimated to be around 0.0010 cm 1. 3. Assignment and analysis of the bands 1,2,5-Selenadiazole is an asymmetric top molecule having an asymmetry parameter j of 0.513; i.e., in the prolate range but relatively far from the prolate symmetric top limit of 1. The rotational levels of the molecule are most conveniently described using the Ir-type of representation, in which we use Ka as the leading K-quantum number. Asymmetry splitting of the rotational levels is described by Kc which takes the values Kc = J Ka and J Ka + 1. The effect of asymmetry splitting is largest for low Ka and decreases with increasing Ka. For our classification of the normal modes of the molecule, we follow the convention that the plane of the molecule coincides with the xz-plane [1,3]. Thus, six of the modes, m1–m6, are totally symmetric (A1) and give rise to infrared active a-type fundamental bands. The m7 and m8 bands are of A2 symmetry and these modes are infrared inactive. The six vibrations m9–m13 are of B1-symmetry giving rise to b-type bands while the out-of-plane modes m14 (B2) and m15 (B2) give rise to c-type bands. The a-type bands are governed by the selection rules where (DKa, DKc) = (even, odd), and the strongest transitions have DKa = 0 and DKc = ± 1. At low spectral resolution such bands are easily recognized by the intense Q-branch structure in the band centre region which is accompanied by P- and R-branch wings showing some clustering of lines spaced by ca. 2C 0.17 cm 1. At high resolution the clusters are resolved, when they consist of a long and intense series of QP- and QR-branch transitions for low Ka having DKa = 0. Each series is dominated by one of the asymmetry split components, namely the component for which DKc = DJ = ± 1. For high Ka, unsplit Q P- and QR-branch series spaced by (B + C) 0.22 cm 1 are the dominant features of the spectrum. For b-type bands, we have the selection rules (DKa, DKc) = (odd, odd), and the most intense transitions have DKa = ± 1 and DKc = ± 1. The spectrum shows an intensity minimum near the band center since the individual Q-branch transitions are spread over a wide wavenumber range. As for a-type bands, cluster structure with spacing of ca. 2C is clearly seen in the P- and R-wings of the spectrum. The strongest cluster lines are for low Ka and consist of the DJ = DKa = DKc = +1 component in the RR-branches and the DJ = DKa = DKc = 1 component in the P P-branches. In addition, weaker cluster lines with DKc = DJ = ± 1, RR- and PP-branch lines appear giving important information on asymmetry splitting. For high
F. Hegelund et al. / Journal of Molecular Spectroscopy 236 (2006) 189–200
Ka, series of unsplit RR- and PP-branch transitions spaced by ca. (B + C) dominate the spectrum. For b-type bands we have also been able to assign a number of Q-branch transitions for low Ka. The selection rules (DKa, DKc) = (odd, even) govern c-type bands which are dominated by an intense central Q-branch. No clustering is apparent in the P- and R-wings of c-type bands since the low-Ka transitions are weak. In the m14 band which is the only c-type band studied in the present paper, short asymmetry split RR- and PP-branch series are assigned for low Ka. For high Ka our assignment consists of long and intense series of this type; the spacing is ca. (B + C). No Q-branch transitions are assigned in m14. To assist and accelerate our assignment of rotational structure in the observed bands, we use ground state combination differences (GSCDs) simultaneously with computer generated Loomis–Wood diagrams. These diagrams are based on our peak lists, and ordered according to upper state quantum numbers, and take into account the GSCDs as originally suggested by Nakagawa and Overend [10]. The assignment program and additional fit programs used in the present work were developed at Aarhus University. We initiated the assignment process with the well resolved and isolated m4 band using GSCDs predicted from the ground state rotational constants of C2H2N280Se [2], and established readily the (J, Ka, Kc) assignment of individual lines in the P- and R-branch clusters for Ka = 0–5. During the process of assignment, the ground state constants were iteratively improved and our assignment was gradually extended to higher Ka. This procedure was continued with the other fundamental bands. A summary is given in Table 2 on some details of our assignments. Typically we assigned around 2000 lines per band, but in the favourable case of m14 nearly 4000 lines were assigned. Asymmetry split lines have been assigned up to Ka 25. From our Loomis–Wood plots it was obvious that the intense line series assigned to C2H2N280Se were accompanied by weaker series showing the same patterns. Since the natural abundance of the 78Se isotope is ca. 50% of the main 80Se isotope, we attempted to assign these series to the C2H2N278Se isotopologue. To prove this assignment, use was made of GSCDs predicted from the ground state rotational constants of C2H2N278Se obtained in [2]. In this way we were able to improve the ground state parameters
191
for this isotopologue, and we finally assigned rotational line series due to C2H2N278Se in all six fundamental bands studied. In most cases the series are rather short and most of them are assigned for low Ka, only. Severe overlap from the series belonging to C2H2N280Se occurs in C2H2N278Se series in many cases. Table 2 also includes details on our assignment of bands for this isotopologue. Typically 500 lines were assigned in each of the bands. The abundance of additional isotopes of Se is below 10%, and we did not attempt to assign lines arising from further isotopologues of 1,2,5-selenadiazole. Such lines are of similar intensity to the hot bands from the low-lying fundamentals m6 and m15 at 489 and 438 cm 1, respectively, in C2H2N280Se [3], but we did not attempt to make assignments of hot bands. The asymmetric rotor Hamiltonian of Watson in A-reduced form assuming Ir-representation [11] was used to fit assigned transitions in the bands studied. In the following section we briefly discuss some details of these fits. Here we give a brief outline of the procedure. From a simultaneous GSCD analysis of the six bands studied and the microwave measurements [2], we obtain the three rotational constants and all five quartic centrifugal distortion constants of the ground state for C2H2N280Se, as shown in Table 3. For C2H2N278Se a similar fit of the data available yields the rotational constants and only one of the centrifugal distortion constants, DJ, as seen in Table 4. For each band we perform upper state analyses using the Watson model. In these fits, the observed wavenumbers are converted to upper state energies as determined from our present ground state constants (column III of Tables 3 or 4). Thus, upper state energies are fitted, but no weighting of the individual energies is introduced. Tables 5 and 6 summarize the resulting upper state parameters obtained for C2H2N280Se and C2H2N278Se, respectively. Lists of assigned observed transitions used in the upper state fits may be obtained from one of the authors (F.H.). 4. Spectroscopic results 4.1. The ground state The present assignment of the m5, m14, m12, m4, m11, and m2 bands of 1,2,5-selenadiazole yields a number of GSCDs from which we may obtain the ground state rotational
Table 2 Details of assignments for 1,2,5-selenadiazole bands Band
Band type
Range of assignments C2H2N280Se
m5 (A1; 726.3 cm 1) m14 (B2; 833.8 cm 1) m12(B1; 880.3 cm 1) m4 (A1; 1005.2 cm 1) m11(B1; 1234.1 cm 1) m2 (A1; 1359.8 cm 1) a
Perturbed.
a in-plane ring bend c out-of-plane CH bend b in-plane ring benda a in-plane CH bend b in-plane CH bend a in-plane ring benda
J < 81 J < 76 J < 79 J < 80 J < 81 J < 78
Ka < 32 Ka < 56 Ka < 57 Ka < 25 Ka < 53 Ka < 23
C2H2N278Se Kc < 80 Kc < 64 Kc < 79 Kc < 79 Kc < 80 Kc < 78
J < 67 Ka < 8 Kc < 6 J < 64 Ka < 51 Kc < 24 J
