Ab initio and DFT conformational study on nitrosamine (H2N–N=O) and N-Nitrosodimethylamine [(CH3)2N–N=O]

Effect of new DFT methods on spectroscopy and NLO analysis of 2-Bromo-5-nitrothiozole and nitrosodimethylamine

The present study reports the effect of new DFT methods introduced in Gaussian 16 viz B2PLYP, B2PLYPD, DSDPBEP86, and MPW2LPLYP on spectroscopy and nonlinear optical (NLO) properties. The 2-Bromo-5-nitrothiozole and nitrosodimethylamine are considered for the study. In addition to new methods, old DFT, MP2, and HF methods are also considered. The spectroscopic study of geometrical parameters, vibrational, electronic absorption spectra, and HOMO-LUMO gap with their molecular orbitals have been reported. The geometrical parameters such as optimized energy, dipole moment, bond length, and angles have been reported. The diffuse and polarized basis set 6-311 +  + G(d,p) has been used for the study. The first- and second-order hyperpolarizabilities have also been reported for both molecules at all methods. This study shows that these two molecules are responsive to nonlinear optical (NLO), which reflects its enhanced applicability in the development of photonic devices. The finite field approach has been applied for calculating first (β) and second hyperpolarizabilities (ϒ) respectively. The HOMO-LUMO energy gap of nitrosodimethylamine is found to be greater than that of 2-Bromo-5-nitrothiozole. Gaussian16 program package has been used for all these calculations.

Detection and structural characterization of nitrosamide H2NNO: A central intermediate in deNOx processes

The structure and bonding of H₂NNO, the simplest N-nitrosamine, and a key intermediate in deNO_x processes, have been precisely characterized using a combination of rotational spectroscopy of its more abundant isotopic species and high-level quantum chemical calculations. Isotopic spectroscopy provides compelling evidence that this species is formed promptly in our discharge expansion via the NH₂ + NO reaction and is collisionally cooled prior to subsequent unimolecular rearrangement. H₂NNO is found to possess an essentially planar geometry, an NNO angle of 113.67(5)°, and a N-N bond length of 1.342(3) Å; in combination with the derived nitrogen quadrupole coupling constants, its bonding is best described as an admixture of uncharged dipolar (H₂N-N=O, single bond) and zwitterion (H₂N⁺=N-O^-, double bond) structures. At the CCSD(T) level, and extrapolating to the complete basis set limit, the planar geometry appears to represent the minimum of the potential surface, although the torsional potential of this molecule is extremely flat.

Effect of methylation on relative energies of tautomers and on the intramolecular proton transfer barriers of protonated nitrosamine: A MR-CISD study

MR-CISD, MR-CISD+Q, and MR-AQCC calculations have been performed on the minima and transition states (corresponding to intramolecular proton transfer between the protonation sites) of the ground state of protonated nitrosamine and N,N-dimethylnitrosamine. Our highest level results (MR-AQCC/cc-pVTZ) for the smaller system indicate that protonation on the N amino (2a) is practically as favorable as the most favorable protonation on the O atom (1a). They also suggest that protonation on the nitroso N atom (2c) is ∼14.5 kcal/mol less favorable than 1a. Results obtained at the MR-CISD+Q/cc-pVTZ level indicate that the effect of methylation on the relative energies of the tautomers is, in order of importance, 2a > 2c and increases their energies by ∼17.5 and 4.8 kcal/mol, respectively. They also indicate that methylation alters significantly the intramolecular proton transfer barriers. The largest differences between the common geometric parameters of both systems have been found for 2a.

Theoretical study of the reaction formalhydrazone with singlet oxygen. Fragmentation of the C=N bond, ene reaction and other processes

Photobiologic and synthetic versatility of hydrazones has not yet been established with (1)O2 as a route to commonly encountered nitrosamines. Thus, to determine whether the "parent" reaction of formalhydrazone and (1)O2 leads to facile C=N bond cleavage and resulting nitrosamine formation, we have carried out CCSD(T)//DFT calculations and analyzed the energetics of the oxidation pathways. A [2 + 2] pathway occurs via diradicals and formation of 3-amino-1,2,3-dioxazetidine in a 16 kcal/mol(-1) process. Reversible addition or physical quenching of (1)O2 occurs either on the formalhydrazone carbon for triplet diradicals at 2-3 kcal mol(-1), or on the nitrogen (N(3)) atom forming zwitterions at ~15 kcal/mol(-1), although the quenching channel by charge-transfer interaction was not computed. The computations also predict a facile conversion of formalhydrazone and (1)O2 to hydroperoxymethyl diazene in a low-barrier 'ene' process, but no 2-amino-oxaziridine-O-oxide (perepoxide-like) intermediate was found. A Benson-like analysis (group increment calculations) on the closed-shell species are in accord with the quantum chemical results.