Ab initio quantum chemical studies on the molecular geometry, force field and vibrational spectra of diborane and related compounds .

Abstract

A good quality force?constant set for a polyatomic molecule can be derived from ab initio calculations using the L?matrix without recourse to arbitrary scale factors or extra computational effort. The RECOVES procedure can be utilized to recover a set of reliable force constants from the ab initio predicted values. It is found that the RECOVES procedure is highly satisfactory for predicting the vibrational frequencies of the isotopomers of the test molecule as well. Comparison of the present force constants with those of previous workers has revealed that the present method mimics the effect of electron correlation, in particular for the C–C/C=C, C=C/C=C, and H–C=C/C=C interaction terms, whereas other similar?level ab initio calculations using scale factors do not show this trend. On the whole, the present force constants are in accord with the general features noted for polyene systems. The RECOVES method is physically more realistic and apparently superior to previous methods of force?constant refinement. We have used here the SCF method with DZ, DZd, DZP, TZP, TZ2P, and TZ2P+f basis sets, and the MP2 method with DZP, TZP, TZ2P, and TZ2P+f basis sets to study the geometry, harmonic vibrational frequencies, and force constants of diborane. An important objective of this study has been to carefully analyze the performance of different basis sets and the effect of electron correlation on the predicted geometrical parameters, force constants, and the derived harmonic frequencies. The results may be useful for further studies on diborane derivatives. Inclusion of polarization functions in the basis set has been found to be very important in the prediction of equilibrium geometry, and the agreement with experimentally derived geometry is satisfactory at the SCF level. However, correlated?level calculations using the MP2 method show further improvement in the calculated B–Ht bond length. For the prediction of harmonic vibrational frequencies by the ab initio methods, the ring region of the diborane molecule behaves differently from the terminal region. The reason for the large deviation of the calculated frequencies from the experimental ones, particularly for the ?7 and ?17 modes, has been analyzed. It appears that single?configuration ab initio calculations may not be adequate for the ring region. The RECOVES force constants obtained from the experimental eigenvalues and the ab initio predicted eigenvectors of the vibrational secular equation are highly satisfactory for reproducing the vibrational frequencies of different isotopomers of diborane. The theoretical vibrational intensities are in good qualitative agreement with the empirically derived experimental band intensities. The SCF/DZP as well as the MP2/DZP methods have been used to study the geometry, barrier to methyl rotation, force constants, and vibrational spectra of methyl diborane and its various deuterated isotopomers. The predicted geometry matches reasonably well with the experimental values. At the theoretical level, slight ring distortion (by 1–2°) occurs as a result of methyl substitution on the terminal position of diborane. With the present vibrational analyses it has been possible to remove the inconsistencies in the previous vibrational assignments. When the basis set is the same, the SCF method overestimates the vibrational frequencies of methyl diborane, whereas the MP2 level—through electron?correlation treatment—shows considerable improvement, except for the ring?stretching vibrations of A? species. The terminal methyl substitution affects the ring vibrations considerably. The BH? stretching frequencies decrease from diborane to methyl diborane. In methyl diborane, weakening of the B–Ht bonds is revealed when compared to diborane. Among the ring?stretching modes, the out?of?phase vibration (?20) increases, whereas the in?phase vibration (?7) decreases by the same extent (?30 cm?¹) upon methyl substitution. Under the rigid?rotor approximation, the methyl torsional barrier was calculated to be 2.0 kcal/mol by both methods, in good agreement with the experimental value. The RECOVES?SCF/DZP force?constant set is almost identical to the RECOVES?MP2/DZP set. The force constants of methyl diborane compare well with those of diborane. The predicted infrared band intensities are in reasonable agreement with the experimental values.