A.K. Chandra

Role of the HNO3↔NOH Isomerization in reactions (i) NH(3Σ−)+O(3P) and (ii) N(4S)+OH(2Π): Ab initio calculations and quantum statistical Rice–Ramsperger–Kassel analysis of the potential energy surfaces

The minimum energy potential energy surfaces for combination of NH(3Σ−)+O(3P) to form HNO(1A’,3A‘) and N(4S)+OH(2Π) to form NOH(3A‘), and the isomerization of HNO(1A’,3A‘) to NOH(1A’,3A‘), decomposition of NOH(1A’,3A‘) to N(4S)+OH(2Π) as well as to H(2S)+NO(2Π) and HNO(3A‘) to H(2S)+NO(2Π) have been obtained by the ab initio methods with geometry optimization at the 6–311G**/MP2=full level with corrections for electron correlation at the MP4SDTQ=full level. At all stationary points on the potential energy surfaces (PES), correction for the zero point vibrational energies are made. The addition reactions to form energized adducts have then been analyzed using a bimolecular version of the quantum statistical Rice–Ramsperger–Kassel (QRRK) theory at different temperatures and pressures. Our analysis predicts that at all temperatures isomerization of HNO(3A‘) to NOH(3A‘) and its reverse isomerization are important. Formation of NO(2Π) in the interstellar clouds can take place from decomposition of NOH(3A‘) as ...

Structure and stability of Li2F

The equilibrium geometries and fundamental vibration frequencies of the Li2F system were calculated by ab initio methods at the MP2 = full/6-311(+ +)G** and CCSD(T) levels. Two isomers were observed and are best described as salts of the Li-2(+) cation with F-. A linear isomer with an arrangement of atoms such as Li-Li-F and a bent C-2v structure are predicted. The stability of these structures are discussed in terms of charge resonance between Li and Li+

Photofragmentation in non-symmetric carbonyl compounds

The energy profiles for radical dissociation of the lowest triplet state of some non-symmetric carbonyls have been examined using the MINDO/3 method and employing configuration interaction. The activation barriers in the cleavage process arise from an avoided crossing between two states of different symmetries. Our analysis reveals that the perpendicular motion of carbonyl carbon is an important component in the reaction coordinate. Selectivity of the cleavage of a \sigma bond adjacent to the chromophore is determined by the third derivative of energy with respect to the reaction coordinate at the starting point of reaction.

Two-dimensional tunneling of hydrogen in a Norrish type II process of a ketone

The two-dimensional tunneling of hydrogen in a Norrish type II process of pentane-2-one is examined through a potential barrier obtained by means of ab initio molecular orbital methods with a

Photochemical intramolecular γ-hydrogen abstraction in a ketone and an alkene and the role of tunneling of hydrogen

6-31G^*^*

Role of free valences and spin densities on the migrating atom in location of saddle points in radical-exchange reactions

basis set and with a correction for electron correlation. The tunneling probability has been estimated using the WKB method with a polynomial function of the energy potential. Two-dimensional calculations lead to a lowering of the tunneling probability obtained by the conventional one-dimensional calculations provided the barrier oscillations are treated as anharmonic.

Further studies on Norrish type II reactions including a reaction in the first excited singlet state and cyclization of 1:4 biradicals

Abstract Intramolecular γ -hydrogen abstraction reactions were examined in pentane-2-one and 2-methyl-1-pentene in their lowest triplet states using the AMI semi-empirical molecular orbital method with the complete geometry optimization in the unrestricted Hartree—Fock frame. The results reveal that the oxygen atom of the carbonyl group and the end carbon atom of the olefinic bond acquire high free valence and spin density indices in their respective lowest triplet states, leading to abstraction of hydrogen from the γ -position relative to the carbonyl and olefinic bonds. The theoretical energy profiles fit with a polynomial and the probability of tunneling of hydrogen was estimated by the WKB (Wentzel, Kramer and Brillouin) method. The results, after thermal averaging of the rate constants, reveal that tunneling of hydrogen is significant at room temperature.

Hydrogen bonding with azabenzenes

Abstract The principle of the conservation of bond orders during radical-exchange reactions is examined using Mayers definition of bond orders. This simple intuitive approximation is not valid in a quantitative sense. Ab initio results reveal that free valences (or spin densities) develop on the migrating atom during reactions. For several examples of hydrogen-transfer reactions, the sum of the reaction coordinate bond orders in the transition state was found to be 0.92 ± 0.04 instead of the theoretical 1.00 because free valences (or spin densities) develop on the migrating atom during reactions. It is shown that free valence is almost equal to the square of the spin density on the migrating hydrogen atom and the maxima in the free valence (or spin density) profiles coincide (or nearly coincide) with the saddle points in the corresponding energy profiles.

Nonperfect synchronization of bond-forming and bond-rupturing processes in the reaction ?? + H2 ? H2 + ??

Abstract The Norrish type II processes of methyl-2,2-dimethyl- cyclopropyl ketone, α-alkoxy acetones and alkyl pyruvates have been examined using the AM1 semi-empirical molecular orbital method with complete geometry optimization at the partial configuration interaction level in the restricted Hartree-Fock (RHF) frame. The results reveal that the methyl-substituted cyclopropyl ketone has a constrained geometry favourable for hydrogen abstraction from the γ-position relative to the carbonyl group in the excited singlet state. The presence of the ether oxygen atom in the β-position relative to the carbonyl group in alkoxy acetones and alkyl pyruvates leads to increased reactivity relative to alkyl monoketones and diketones respectively. The cyclization of 1:4 biradicals has been studied in the unrestricted Hartree-Fock (UHF) frame, and the results reveal that the 1:4 biradical derived from alkoxy acetones readily cyclizes to form oxetanols. On the other hand, in the 1:4 biradicals derived from methyl-substituted cyclopropyl ketone, the three-membered ring breaks readily to form an enol intermediate. Delocalization of an odd electron in 1:4 biradicals derived from alkyl pyruvates is thought to make cyclization difficult.

An investigation of bond formation in the weakly bound first excited 1Σ and lowest 3Σ states of HeH

Results of infrared spectroscopic studies following the standard procedure are reported for the hydrogen-bonding systems involving phenol–azabenzene and ethanol–azabenzene complexes. The theoretical model proposed by Bader is examined and a theoretical index for an assessment of the relative strength of these hydrogen-bonding bases is proposed.