Electronic structure and spectroscopy properties of some thicarboy compounds

Abstract

8.1 SEMI-EMPIRICAL PARAMETERS Different sets of bonding parameters in the CNDO/2 formalism and various approximations for evaluating the off-diagonal matrix element in the SCC-EH method were employed for a group of thioamides and related molecules so as to know about their effect on the results derived. As far as the electronic charge distribution and the orbital energies obtained from the different versions of the CNDO/2 and SCC-EH methods are concerned, a qualitative agreement between the results in each group was noted. Even between the CNDO/2 and SCC-EH results a qualitative agreement was observed. In fact, this agreement between the different versions of the CNDO/2 and SCC-EH methods within themselves gives one a choice to any version of the methods for application to thioamide molecules. However, certain points could be made here. The approximation suggested by the author for the off-diagonal matrix element in the SCC-EH treatment, though it may not be superior to the earlier approximations, gave charge distribution and orbital energies comparable to the Wolfsberg–Helmholz [25] approximation. Further, the use of Cusachs [7] type approximation for the bonding parameters in the CNDO/2 method yields results comparable to those given by Clark [36]. Thus, from the present work, for quantum chemical study of the electronic structure of thioamides, in the SCC-EH method, for obtaining the off-diagonal Hamiltonian matrix element, either the Wolfsberg–Helmholz expression or the expression suggested by the author, and in the CNDO/2 method, Clark’s [35] approximation for the bonding parameters may be more suitable. For comparison, calculations with two basis sets-namely, Burns’ single?exponent and Clementi’s multiexponent basis sets-were carried out for the simplest thioamide, thioformamide. The improvement in the results (orbital energies) was not significant enough to justify the greater computational time such a calculation may require.

8.2 ELECTRONIC STRUCTURE All the molecules studied, except methyl xanthate and methyl trithiocarbonate, contain a thiocarbonyl group adjacent to a nitrogen atom containing a ? lone pair. The ? bond orders obtainable from the CNDO/2 method and the ? overlap population from the SCC-EH treatment may be taken as a measure of the ??bond nature of the concerned bond. The ? bond orders (from CNDO/2(III)) and the ? overlap populations (from SCC?EH(II)) of some molecules are given in Table 8.1. From these values, it may be inferred that in thioamide-like molecules, the C=S moiety has less double-bond character than is indicated by the structural formula, and the C–N group has acquired partial double?bond character. This inference was earlier indicated from vibrational spectra and other studies [127–131]. The appearance of the C=S stretching frequency in thioamide and thiourea type compounds around 700–800 cm?¹ reveals reduced double?bond character. The relatively low stretching force constants (3.3 to 4.0 mdyn/Å) found in normal coordinate calculations support this view. X?ray structural analysis also reveals reduced double?bond character in the C=S group. A value around 0.170 nm is quoted for the C=S bond length [255, 250], larger than the expected 0.160 nm for a true C=S double bond. The less effective ? overlap between the 2p orbital of carbon and the 3p orbital of sulfur explains this reduction. The C=S group may show even less double?bond character when bonded to atoms capable of strong mesomeric electron release, e.g., nitrogen. The partial double?bond nature of the C–N moiety manifests in the barrier to internal rotation (15–30 kcal mol?¹), as revealed by NMR studies [253, 323]. Dithiobiuret, monothiobiuret, monothiodiacetamide, and guanyl thiourea show from ? bond orders (or ? overlap populations) that the –NH– moiety prevents extended conjugation. Dithioxamide, however, which lacks –NH– groups, has completely extended conjugation. In the N?methyl derivatives of TF, TA, and TU, the increase in ? bond nature of the C–N bond and the decrease in ? bond nature of the C=S bond with increasing methylation are clearly seen from the ? bond orders. This is explained as a mesomeric effect of the methyl groups. From the CNDO/2 results, major contribution of the polar resonance structure in DMDTC and lesser contribution in MXN and MTTC can be inferred, in agreement with vibrational and X?ray studies. In all molecules considered, nitrogen atoms bearing lone pairs behave as ? donors and ? acceptors, while thiocarbonyl sulfur behaves as a ? acceptor and ? donor, consistent with formation of transition?metal complexes. The imino nitrogen atom of GTU was found to be a strong ? acceptor and strong ? donor. Proton electron densities relate qualitatively to proton chemical shifts.

8.3 DIPOLE MOMENTS Dipole moments were calculated by CNDO/2 and SCC?EH methods to compare with experimental values. Fair agreement was found.

8.4 PHOTOELECTRON SPECTRA The SCC?EH and CNDO/2 methods give orbital energies in better agreement with photoelectron spectral bands (via Koopmans’ theorem) than conventional CNDO/2(0) and CNDO/S methods. For interpretation of photoelectron spectra, the present CNDO/2 and SCC?EH versions-especially CNDO/2(III) and SCC?EH(II)-may be preferred. Substitution of hydrogen by electron?releasing groups (–NH?, –CH?) increases the energy of occupied orbitals, as seen in N?methylation of TA and TU.

8.5 ELECTRONIC SPECTRA The PPP?LCI method was mainly used to interpret electronic spectra. SCC?EH and CNDO/2 results sometimes supplemented this. Assignments were made for TA, TU, their N?methyl derivatives, and TSC, TCH, DTO, DTB, MTB, MTD, GTU, DMDTC, MXN, and MTTC. Several earlier controversies were resolved. Interaction between C=S and C=O clouds (as in MTB or MTD) or between two C=S chromophores (as in DTB or DTO) is reflected in spectra. The order of interaction was: DTO > DTB > MTB > MTD. Electronic transitions in DTO are mainly intramolecular charge?transfer, while DTB shows both charge?transfer and localized transitions. Use of CNDO/2?derived core charges in PPP?LCI reproduced spectra satisfactorily.

8.6 CONFORMATIONAL ANALYSIS CNDO/2(0) and SCC?EH(II) methods were used. Conformational stabilities of N?methyl thioformamide, N?methyl thioacetamide, and N,N??dimethyl thiourea were examined. N?methyl thiourea is more stable in the trans form; the cis orientation of the methyl proton is more favourable. MTB is more stable in the cis?trans –CONHCS– form according to both methods, supported by NMR. For MTD, CNDO/2(0) suggested equal likelihood of cis?trans and trans?trans forms, while SCC?EH(II) strongly favoured cis?trans. Vibrational studies also support this ambiguity. For GTU, a trans?trans –CSNHC=NH– form is relatively stable. CNDO/2 predicts a small energy difference (~6 kcal mol?¹), suggesting possible coexistence of both forms.