QUANTUM CHEMICAL STUDY OF THE GAS-PHASE DECAY OF CATION RADICALS 4,4-METHYL-2-PENTANONE AND METHYL-3,3-DIMETHYLBUTANOATE
Rubrics: 1. CHEMISTRY
Authors:
1.
KNITU
Type:
Article
Pages:
from 14
to 17
Status:
Published
Received:
27.01.2026
Accepted:
28.01.2026
Published:
28.01.2026
Language:
Russian
Keywords:
MCLAFFERTY REARRANGEMENT, 4,4-DIMETHYL-2-PENTANONE, MERTHYL-3,3-DIMETHYLBUTANOATE, QUANTUM CHEMICAL CALCULATION, MP2 METHOD
Abstract:
The McLafferty rearrangement is observed in mass spectrometry at the stage of fragmentation of aldehydes, ketones and other compounds containing C=O, C=C groups, as well as an H atom in the γ-position relative to such a group. The emergence of research using the method of ultrafast destructive probing of cation-radicals with femtosecond measurements has generated renewed interest in the McLafferty rearrangement. A theoretical study of the fragmentation and McLafferty rearrangement reactions of the cation-radicals of methyl 3,3-dimethylbutanoate and 4,4-dimethyl-2-pentanone was carried out. Using the UMP2/6-31+g(d,p) method, the transition states for these reactions were found, and descents along the reaction coordinate were performed. A comparison of the obtained theoretical data with the recently published results of femtosecond measurements for the cation-radical of 4,4-dimethyl-2-pentanone [1] was carried out. For 4,4-dimethyl-2-pentanone, a fast step of γ-hydrogen transfer of the 4,4-dimethyl-2-pentanone cation radical is observed, forming a six-membered transition state. According to the UMP2/6-31+g(d,p) method, the activation energy of this reaction is 2.1 kJ/mol. For both cation radicals, it was possible to localize the transition state of the second step of the McLafferty rearrangement, but the activation energy of these reactions is an order of magnitude higher than that of the first reactions. According to a mass spectrometric study, the second step for the 4,4-dimethyl-2-pentanone cation radical proceeds with the formation of the propen-2-ol cation radical and 1,1-dimethylethylene. According to theoretical studies, the activation energy of this reaction is 64.64 kJ/mol. The transition state for the fragmentation reaction of 4,4-dimethyl-2-pentanone at the C3-C4 carbon bond was localized, resulting in the formation of a 2-oxopropyl radical and a tert-butyl cation. Calculations showed that the first step of the McLafferty reaction for the methyl 3,3-dimethylbutanoate radical cation proceeds through a six-membered cyclic transition state. According to the UMP2/6-31+g(d,p) method, the activation enthalpy for this reaction is 2.3 kJ/mol.
The McLafferty rearrangement is observed in mass spectrometry at the stage of fragmentation of aldehydes, ketones and other compounds containing C=O, C=C groups, as well as an H atom in the γ-position relative to such a group. The emergence of research using the method of ultrafast destructive probing of cation-radicals with femtosecond measurements has generated renewed interest in the McLafferty rearrangement. A theoretical study of the fragmentation and McLafferty rearrangement reactions of the cation-radicals of methyl 3,3-dimethylbutanoate and 4,4-dimethyl-2-pentanone was carried out. Using the UMP2/6-31+g(d,p) method, the transition states for these reactions were found, and descents along the reaction coordinate were performed. A comparison of the obtained theoretical data with the recently published results of femtosecond measurements for the cation-radical of 4,4-dimethyl-2-pentanone [1] was carried out. For 4,4-dimethyl-2-pentanone, a fast step of γ-hydrogen transfer of the 4,4-dimethyl-2-pentanone cation radical is observed, forming a six-membered transition state. According to the UMP2/6-31+g(d,p) method, the activation energy of this reaction is 2.1 kJ/mol. For both cation radicals, it was possible to localize the transition state of the second step of the McLafferty rearrangement, but the activation energy of these reactions is an order of magnitude higher than that of the first reactions. According to a mass spectrometric study, the second step for the 4,4-dimethyl-2-pentanone cation radical proceeds with the formation of the propen-2-ol cation radical and 1,1-dimethylethylene. According to theoretical studies, the activation energy of this reaction is 64.64 kJ/mol. The transition state for the fragmentation reaction of 4,4-dimethyl-2-pentanone at the C3-C4 carbon bond was localized, resulting in the formation of a 2-oxopropyl radical and a tert-butyl cation. Calculations showed that the first step of the McLafferty reaction for the methyl 3,3-dimethylbutanoate radical cation proceeds through a six-membered cyclic transition state. According to the UMP2/6-31+g(d,p) method, the activation enthalpy for this reaction is 2.3 kJ/mol.
Keywords:
MCLAFFERTY REARRANGEMENT, 4,4-DIMETHYL-2-PENTANONE, MERTHYL-3,3-DIMETHYLBUTANOATE, QUANTUM CHEMICAL CALCULATION, MP2 METHOD
MCLAFFERTY REARRANGEMENT, 4,4-DIMETHYL-2-PENTANONE, MERTHYL-3,3-DIMETHYLBUTANOATE, QUANTUM CHEMICAL CALCULATION, MP2 METHOD
