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Through bond and through space interactions in dehydro- diazine radicals: A case study of 3c-5e interactions


Owing to the 3c-5e (3-centred – 5-electrons) interactions between two nitrogen lone pairs and a radical electron, the dehydrodiazine radical isomers are very interesting in the fundamental point of view. Among them, pyrimidine has three (1a-1c), pyridazine has two (2a and 2b) and pyrazine has only one (3a) radical isomers. Based on quantum chemical calculations at (U)B3LYP, (U)M06-2X, (U)BLYP, CBS-QB3 and (U)CCSD(T) levels with cc-pVTZ as the basis set, we found out 4-dehydropyrimidine (1b) radical as the most stable isomer among the three pyrimidine radicals, followed by 2- dehydropyrimidine (1a) and 5-dehydropyrimidine (1c) radical isomers. In the case of pyridazine, 3-dehydro radical isomer (2a) is more stable than 4-dehydropyridazine (2b). Bond dissociation energy (BDE) calculations and estimation of radical stabilization energies (RSE) using isodesmic reactions revealed the stability order among the six isomeric diazine radicals as 1c < 2b < 2a < 1a < 3a < 1b. Spin densities at each radical centre and non-zero values at nitorogen centres provided the information about the extent of delocalization of radical electon, which was consistent with the relative stabiliy order of all the isomers. Multiconfigurational CASSCF and natural bond orbital (NBO) calculations suggested the presence of direct through space interactions (between lone pairs and radical, TS) that plays a dominant role over the through bond (through intervening bonds, TB) interactions in deciding the stability order. To confirm these results, we have also estimated the proton affinities (PA) for each nitrogen atom and compared them with their respective parent diazines, where the lowering in the PA values convincingly envisaged the extent and strength of interactions between the nitrogen and radical centre. Atoms-in-molecules (AIM) analysis and estimation of hyperfine coupling constants have also been performed to verify these results. All these results showed that the through space interaction between the lone pair and the radical electron is very important for the electronic structural and stability aspects in dehydrodiazine radicals.

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Publication details

The article was received on 09 Nov 2017, accepted on 21 Dec 2017 and first published on 03 Jan 2018

Article type: Paper
DOI: 10.1039/C7CP07579K
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Through bond and through space interactions in dehydro- diazine radicals: A case study of 3c-5e interactions

    M. Saraswat and S. Venkataramani, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C7CP07579K

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