Ab initio HF/6-31+G*, MP2/6-31+G*, B3LYP/6-31+G* level calculations have been performed on HSe-NH₂ to estimate the Se-N rotational barriers and N-inversion barriers. Two conformers have been found withsyn andanti arrangement of the NH₂ hydrogens with respect to Se-H bond. The N inversion barriers in selenamide are 1.65, 2.47, 1.93 kcal/mol and the Se-N rotational barriers are 6.58, 6.56 and 6.12 kcal/mol respectively at HF/6-31+G*, MP2/6-31+G* and B3LYP/6-31+G* levels respectively. The n_N →Σ *_Se-H negative hyperconjugation is found to be responsible for the higher rotational barriers.

Ab initio and density functional calculations have been performed on the different possible structures of selenourea(su), urea(u) and thiourea(tu) to understand the extent of delocalisation in selenourea in comparison to urea and thiourea. Selenourea(su-1) withC₂ symmetry has the minima on the potential energy surface at MP2(fu)/6-31+G* level. The C-N rotational barrier in selenourea is 8.69 kcal/mol, which is 0.29 and 0.11 kcal/mol more than that of urea and thiourea respectively at MP2(fu)/6-31+G* level. N-inversion barrier is 0.55 kcal/mol at MP2(fu)6-31+G* level. NBO analysis has been carried out to understand the nature of different interactions responsible for the electron delocalisation.

Halogen bonding interactions of type X$\cdots$O=C are important in various fields including biological systems. In this work, theoretical calculations were carried out using B3LYP/6-31++G^∗∗, MP2/6-31++G^∗∗ and MP2/aug-cc-pVDZ methods on a series of O$\cdots$X halogen bonds between CH₂O andCH₃CHO as halogen bond acceptor with X-Y (X = Cl, Br; Y = CF₃, CF₂ H, CFH₂, CN, CCH, CCCN) as halogen bond donors. The strength of interaction energy for O$\cdots$Br halogen-bonded complexes varies from −2.16 to −5.26 kcal/mol while for O$\cdots$Cl complexes, it is between −1.65 to −3.67 kcal/mol, which indicate the O$\cdots$Br bond to be stronger in comparison to O$\cdots$Cl bond. SAPT analysis suggests that the strength of halogen bonding arises from the electrostatic and induction forces while dispersion is playing a comparatively smaller role. The halogen-bonded interaction energies were found to correlate well with positive electrostatic potential V$_{\text{S,max}}$, halogen bonded distances, and the change in s-character of C-X bond. The halogen-bonded interaction energies were also evaluated for O$\cdots$I bonded complexes and thus these complexes were found to be stronger than O$\cdots$Br and O$\cdots$Cl bonded complexes.

Ab initio and DFT methods have been employed to study the hydrogen bonding ability of formamide, urea, urea monoxide, thioformamide, thiourea and thiourea monoxide with one water molecule and the homodimers of the selected molecules. The stabilization energies associated with themonohydrated adducts and homodimers’ formation were evaluated at B3LYP/6-311++G^{\ast\ast}$ and MP2/6-311++G^∗∗ levels. The energies were corrected for zero-point vibrational energies and basis set superposition error using counterpoise method. Atoms in molecules study has been carried out in order to characterize the hydrogen bonds through the changes in electron density and laplacian of electron density. A natural energy decomposition and natural bond orbital analysis was performed to understand the nature of hydrogen bonding.

In this work, density functional theory and ab initio molecular orbital calculations were used to investigate the hydrogen bonded complexes of type RCHO···HOR′(R = H, CN, CF₃, OCH₃, NH₂; R′ = H, Cl, CH₃, NH₂, C(O)H, C₆H₅) employing 6-31++g^** and cc-pVTZ basis sets. Thus, the present work considers how the substituents at both the hydrogen bond donor and acceptor affect the hydrogen bond strength. From the analysis, it is reflected that presence of –OCH₃ and –NH₂ substituents at RCHO greatly strengthen the stabilization energies, while –CN and –CF₃ decrease the same with respect to HCHO as hydrogen bond acceptor. The highest stabilization results in case of (H₂N)CHO as hydrogen bond acceptor. The variation of the substituents at –OH functional group also influences the strength of hydrogen bond; nearly all the substituents increase the stabilization energy relative to HOH. The analysis of geometrical parameters; proton affinities, charge transfer, electron delocalization studies have been carried out.

The hydrogen bonded complexes of serine with water and with H₂O₂ (HP) have been completely investigated in the present study using second-order Møller-Plesset perturbation theory (MP2) and density functional theory (DFT) in order to determine their geometries, stabilization energies, vibrational frequenciesand electronic characteristics. The stabilization energies (∆E_BSSE) span a range of − 2.76 to − 12.46 kcal/mol for 1:1 serine–water complexes and − 4.54 to − 12.73 kcal/mol for 1:1 serine–HP complexes. The ∆E_BSSE values suggest that serine–HP complexes are more stable than serine–water complexes. For all the structures, complex formation results in elongation of N-H, O-H bonds and shortening of C-H bonds thereby showing the red-shift and blue-shift for the respective bonds. The structural, vibrational and electronic features are in accordance with the fact that HP is a better proton donor and water a better proton acceptor. The excellent relationship is obtained for the variation of ∆E_BSSE values with the sum of the ρ values and the sum of laplacian at the BCPs for the HBs. The E ⁽²⁾ values are also in concordance with the calculated ∆E_BSSE values.