TY - JOUR

T1 - Stabilization of Saturated Carbocations in Condensed Phases

AU - Stoyanov, Evgenii S.

PY - 2017/12/21

Y1 - 2017/12/21

N2 - Based on the experimentally established mechanism of hyperconjugative stabilization of the simplest saturated carbocations [Stoyanov, E. S.; et al. PCCP, 2017, 19, 7270], the infrared spectra of t-alkyl+ and methyl-cyclo-pentyl+ carbocations were interpreted. This approach allows us to extract new information about the electronic state of (CH3)2C+R cations with R = H, CH3, C2H5, C4H7, and CH(CH3)2, namely, the electron density distribution over the (CH3)2C group and the positive charge dispersion on the H atoms of this group. Thus, donation of the electron density to the empty 2pz orbital of the sp2 C atom occurs not only from one C-H bond oriented parallel to the 2pz orbital but also equally from all other C-H and C-C bonds of the molecular group involved in hyperconjugation. This mechanism preserved the isoelectronic nature of this group toward the corresponding groups of the neutral alkanes. Hyperconjugation and polarization are closely linked in stabilization of carbocations: the strengthening of one effect weakens the second and vice versa without changing the efficiency of scattering of the positive charge in the carbocation. In the condensed phase, carbocations are additionally stabilized by the bulk effect and hydrogen bonding with the environment: increasing H-bonding strength increased hyperconjugation and decreased polarization. The contribution of all the effects on the stabilization of carbocations was evaluated.

AB - Based on the experimentally established mechanism of hyperconjugative stabilization of the simplest saturated carbocations [Stoyanov, E. S.; et al. PCCP, 2017, 19, 7270], the infrared spectra of t-alkyl+ and methyl-cyclo-pentyl+ carbocations were interpreted. This approach allows us to extract new information about the electronic state of (CH3)2C+R cations with R = H, CH3, C2H5, C4H7, and CH(CH3)2, namely, the electron density distribution over the (CH3)2C group and the positive charge dispersion on the H atoms of this group. Thus, donation of the electron density to the empty 2pz orbital of the sp2 C atom occurs not only from one C-H bond oriented parallel to the 2pz orbital but also equally from all other C-H and C-C bonds of the molecular group involved in hyperconjugation. This mechanism preserved the isoelectronic nature of this group toward the corresponding groups of the neutral alkanes. Hyperconjugation and polarization are closely linked in stabilization of carbocations: the strengthening of one effect weakens the second and vice versa without changing the efficiency of scattering of the positive charge in the carbocation. In the condensed phase, carbocations are additionally stabilized by the bulk effect and hydrogen bonding with the environment: increasing H-bonding strength increased hyperconjugation and decreased polarization. The contribution of all the effects on the stabilization of carbocations was evaluated.

KW - STABLE CARBONIUM-IONS

KW - TERT-BUTYL CATION

KW - PROTONATION

KW - SPECTRA

KW - FORCE

UR - http://www.scopus.com/inward/record.url?scp=85038958453&partnerID=8YFLogxK

U2 - 10.1021/acs.jpca.7b10068

DO - 10.1021/acs.jpca.7b10068

M3 - Article

C2 - 29220179

AN - SCOPUS:85038958453

VL - 121

SP - 9638

EP - 9644

JO - Journal of Physical Chemistry A

JF - Journal of Physical Chemistry A

SN - 1089-5639

IS - 50

ER -