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Rotational transitions of several hydrogen-bonded complexes between formic acid and water have been observed with a pulsed nozzle Fourier transform microwave spectrometer between 8 and 26 GHz. Three sets of rotational transitions have been assigned with the help of their Stark effects and of microwave–microwave double resonance experiments to formic acid–water, formic acid–(water)2 and (formic acid)2–water. Rotational constants and some centrifugal distortion constants have been fitted for each complex, and the components of the permanent electric dipole moments have been determined from Stark splittings. Structures and binding energies from ab initio calculations have been determined to the observed formic acid–water complexes.

We have carried out a detailed investigation of the nature of the π-H interaction in the ethene–H2O, benzene–H2O, and benzene–(H2O)2 complexes using large basis sets (ranging from 6-31+G* to TZ2P++) and high levels of theory. The minimum geometries, and hence the vibrational frequencies, of all the complexes have been obtained at the second order Mo/ller–Plesset (MP2) level of theory. The binding energy of the ethene–H2O complex is only about 1 kcal/mol lower than that of the benzene–H2O complex. In the benzene–(H2O)2 complex, the interaction of benzene with the π-bonded water to that with the second water is nearly equivalent. In order to explain the above interesting facets of the interaction of water with benzene and ethene, the interaction energies were decomposed into the individual interaction energy components using the recently developed symmetry adapted perturbation theory (SAPT) program. The SAPT results indicate that the repulsive exchange energies play a crucial role in governing the energies and geometric preferences of these complexes. A detailed analysis of the vibrational frequencies of these complexes has also been done to examine the changes in the frequencies of the monomers upon complexation. It is found that changes in the out-of-plane bending frequencies of benzene and ethene can be correlated to the interaction energies of these complexes, in particular the exchange energy.

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Ab initio molecular orbital calculations have been used to study the decomposition of methyl azide (CH 3 N 3 ), methanimine, and its isomers (CH 3 N) in both lowest lying singlet and triplet states. Geometries were optimized using UMP2/6-31G(d,p) level of theory while energies of the stationary points on potential energy surfaces were obtained from QCISD(T) calculations with larger 6-311++G(d,p) and 6-311++G(3df,2p) basis sets and corrected for zero-point energies. The temperature dependence of the rate constants of various dissociative processes has also been calculated using the conventional transition-state theory. While the decomposition of methyl azide occurs, in the singlet state, through a concerted motion of N 2 elimination with hydrogen shift, giving methanimine, the triplet methyl azide does not exist as a discrete species but falls apart, giving triplet methylnitrene plus N 2 . Starting from singlet methanimine, 1,1-H 2 elimination giving HNC is found to be favored over 1,2-H 2 elimination giving HCN, a 1,2-H shift yielding aminocarbene, and N-H bond cleavage producing the H 2 CN radical. The hot HNC molecule is expected to rearrange rapidly to HCN. From singlet aminocarbene (HCNH 2 ), 1,2-H 2 loss giving HNC is also a less energy-demanding step than the 1,2-H 2 loss, generating HCN. Overall, it appears that, in the lowest singlet state, HCN is not directly formed upon fragmentation of methanimine but rather from rearrangement of HNC which is the primary product. In the triplet state, the HCN formation from either methylnitrene or methanimine passes through successive losses of H atoms.