Asit K. Chandra

A density functional study of weakly bound hydrogen bonded complexes

Abstract Density functional theory (DFT) calculations with B3LYP exchange-correlation functional and using 6-31++G(d,p) basis functions have been performed on weakly bound hydrogen bonded complexes between HX (X=F,Cl) and alkenes and alkynes, such as C 2 H 4 , C 2 HX (X=H,F,Cl), C 4 H 2 and allene. Calculations have also been carried out at MP2=full level of theory and using the same basis set as mentioned above for comparison with the DFT results. It has been observed that the BSSE uncorrected binding energies obtained from the B3LYP calculations are always lower than the corresponding MP2 results whereas opposite trend has been observed after BSSE correction. Hydrogen bond lengths obtained from MP2 and B3LYP calculations differ insignificantly. The H-X frequency shift due to complex formation has been well reproduced by the B3LYP method.

Theoretical study of the protonation and deprotonation of cytosine. Implications for the interaction of cytosine with water

The geometries, harmonic vibrational frequencies and energies of the two stable cyclic structures of the keto tautomer of cytosine complexed with water are computed using density functional theory (B3LYP) combined with the 6-3111G(d,p) basis set. The effect of complex formation with water on the pyramidalization of the amino group is discussed. The proton affinities of the oxygen and nitrogen atoms and the deprotonation enthalpies of the three NH bonds of cytosine are computed at the same level of theory. The deprotonation enthalpies of the two NH bonds of the amino group differ by 23 kJ mol 21 from each other and this reflects the asymmetric deformation of the amino group. The most stable hydrogen bond between cytosine and water is formed at the acceptor atom characterized by the lowest proton affinity and at the NH group having the highest acidity. The results are compared with data obtained at the same level of theory for the uracil‐ and thymine‐water complexes. For the three nucleobases, the intermolecular distances and the energies of the hydrogen bonds formed at the different sites depend on the proton affinity and the deprotonation enthalpy of these sites. The dominance of the proton donor capacity in determining the hydrogen-bond energies and the cooperativity in the cyclic structures are discussed. q 2000 Elsevier Science B.V. All rights reserved.

Approach to regiochemistry using local softness in 1,3‐dipolar cycloadditions

The principle of hard and soft acids and bases is applied in the local sense to rationalize the regiochemistry in the cycloaddition reactions of a few typical 1,3‐dipoles, in particular those with phosphorus‐containing dipolarophiles. Local softnesses are calculated using density functional theory. It is observed that the regioselectivity can be explained using these new reactivity descriptors based solely on the properties of the reactants.u2003© 1998 John Wiley & Sons, Inc.u2003J Comput Chem 19: 195–202, 1998

Quantum chemical study of hydrogen-bonded CH2CO…HX and CH2CCH2…HX (X = Cl, F) complexes

Abstract An ab initio quantum chemical study, largely at the 6–311 + + G ∗ ∗ / MP 2 = full level of theory has been carried out on the 1:1 and 2:1 complexes of HX (X = Cl, F) with ketene and are compared with the corresponding isoelectronic complexes of H2Cue5fbCue5fbCH2 and HX. Three possible modes of the hydrogen bond between HX and ketene via the dipole-induced dipole (X-H…C, X-H…O) and the weak X-H…π interactions were considered. The binding energy has been calculated after giving careful consideration to basis set superposition effects and zero-point vibrational energy effects. The electron correlation correction to the binding energy has also been computed. Weak X-H…π hydrogen bonds are perhaps possible with nonpolar Cue5fbC and Cue5fbC pi-clouds.

Density functional calculations on simple carbonyl bases: protonation and hydrogen bond formation with water

Abstract Density functional theory (B3LYP) calculations combined with the 6-31++G(d,p) basis set have been carried out on protonated carbonyl bases RHCue605O (R=F, CH 3 , NH 2 ) and R 2 Cue605O (R=F, H, CH 3 ). The substituent effects on the equilibrium structures and vibrational frequencies of protonated bases are discussed. Protonation results in spectacular changes of the CF and CH bond lengths and the frequencies and intensities of the ν (CF) and ν (CH) stretching vibrations. These features are discussed in terms of the lone pair effect. Correlations between the ν (OH) and ν (CH) stretching frequencies and the corresponding OH and CH distances are presented. The relative changes of distances and angles are on the average 10 times higher for protonation than for hydrogen bond formation. The hydrogen bond energies are linearly correlated to the proton affinities of the corresponding sites and positive departures from the linearity are observed for closed dimers where the distance between the hydrogen atom of the substituent and the O atom of water is lower than 2.5 A. Cooperativities evaluated from infrared frequency shifts are different and seem to be more sensitive to the angular properties.

New look at free radical addition to olefins using local reactivity indices

Calculations show that the regioselectivity of free radical naddition nto olefins can be rationalized by using local reactivity indices derived nfrom density functional theory, such as the condensed Fukui functions nand atomic softness.

Theoretical investigations of the gas-phase pre-reactive complexes of oxirane with HF, HCl, F2 and ClF

Abstract The pre-reactive complexes formed by oxirane (CH 2 ) 2 O and diatomic halogens (XY, X=H, F and Y=F, Cl) were investigated through ab initio calculations at the MP2=full level using 6-311++G(d,p) basis functions. While in the case of oxirane and HX (X=F, Cl) rather strong hydrogen bonds form between the oxygen atom of oxirane and H atom of HX, the interaction is weaker in the case of F 2 . Significant changes in monomer geometries have been observed, leading to a large low-frequency shift of the XY vibration. Energy partitioning analysis of the total interaction energy has been performed using symmetry adapted perturbation theory to probe the nature of the forces involved in the complexes and how they differ on the change of X and Y atoms.

THEORETICAL STUDIES ON THE H2O.CIF COMPLEX

Abstract Geometry optimizations at the MP2 level with 6–31++G(d,p) basis functions and single-point calculations at the QCISD(T) leve with 6–31++G(3df,2p) basis functions have been carried out tot determine the structures and binding energies of the complexes of the molecules H 2 O and ClF. The binding energy of the most stable complex is found to be 4.05 kcal/mol at the QCISD(T) level of theory.

Theoretical Investigation of the Cooperativity in CH3CHO·2H2O, CH2FCHO·2H2O, and CH3CFO·2H2O Systems

The hydrogen bond interaction between CH3CHO, CH2FCHO, and CH3CFO and two water molecules is investigated at the B3LYP/6-311