Hyperconjugative interactions in vinylic systems: the problem of the barrier to methyl rotation in acetone

THE ORIGIN OF THE ROTATIONAL BARRIER IN DIMETHYL ETHER AND DIMETHYL SULFIDE: A THEORETICAL STUDY

The torsional potential function for methyl rotation in dimethyl ether (DME) and dimethyl sulfide (DMS) has been determined by utilizing ab initio (Hartree–Fock and MP2) and density functional theory (B3LYP, B3P86, and B3PW91) methods along with several basis sets. Natural bond orbital (NBO) analysis was also applied to investigate the origin of the rotational barrier.

Origin of methyl torsional barrier in 1-methyl-2(1H)-pyridinimine and 3-methyl-2(1H)-pyridone: II. Ground state

To get the insight into the electronic structure-methyl torsion correlation in nitrogen heterocyclic molecules, a comparative study on torsion of the methyl group in 1-methyl-2(1H)pyridone (1MPY), 1-methyl-2(1H)pyridinimine (1MPI), and 3-methyl-2(1H)pyridone (3MPY) was carried out using ab initio calculations. To understand the barrier forming mechanism in the ground state and its consequence on the molecular structure, the ground state torsional potential has been investigated by partitioning the barrier energy using the natural bond orbital (NBO) theoretical framework. The NBO analysis reveals that the delocalization energy is the barrier forming term whereas the Lewis energy is always antibarrier for all these molecules. To get further insight into the effect of local electronic structure on the methyl torsional barrier, the individual bond-antibond interactions and structural energy contributions have been investigated. It was found that when the bond order difference between the vicinal bonds does not change appreciably during the course of methyl rotation, the local electronic interactions with the methyl group do not play any decisive role in barrier formation as observed in the case of 1MPY and 1MPI. In these cases, it is the skeletal relaxation during methyl rotation that plays an important role in determining the barrier. On the other hand, if the bond order change is appreciable as is the case for 3MPY, the local interactions alone suffice to describe the origin of the torsional barrier of the methyl group.

Origin of threefold symmetric torsional potential of methyl group in 4-methylstyrene

To understand the effect of the para position vinyl group substitution in toluene on methyl torsion, we investigated 4-methylstyrene, a benchmark molecule with an extended pi conjugation. The assignment for a 33 cm(-1) band in the excitation spectrum to the 3a(2) torsional transition, in addition to the assignments suggested previously for the other bands in the excitation spectrum, leads to the model potentials for the ground as well as excited states with V(3) (")=19.6 cm(-1), V(6) (")=-16.4 cm(-1) and V(3) (')=25.6 cm(-1), V(6) (')=-30.1 cm(-1), respectively. These potentials reveal that both in ground and excited states, the methyl group conformations are staggered with a 60 degrees phase shift between them. MP2 ab initio calculations support the ground state conformations determined from experiments, whereas Hartree-Fock calculations fail to do so. The origin of the modified ground state potential has been investigated by partitioning the barrier energy using the natural bond orbital (NBO) theoretical framework. The NBO analysis shows that the local delocalization (bond-antibond hyperconjugation) interactions of the methyl group with the parent molecule is sixfold symmetric. The threefold symmetric potential, on the other hand, stems from the interaction of the vinyl group and the adjacent ring pi bond. The threefold symmetric structural energy arising predominantly from the pi electron contribution is the barrier forming term that overwhelms the antibarrier contribution of the delocalization energy. The observed 60 degrees phase shift of the excited state potential is attributed to the pi(*)-sigma(*) hyperconjugation between out of plane hydrogens of the methyl group and the benzene ring.

NMR Spectroscopy Investigation of the Cooperative Nature of the Internal Rotational Motions in Acetophenone

The proton NMR spectrum of the doubly enriched acetophenone-carbonyl,methyl-13C2 isotopomer dissolved in a liquid-crystalline solvent (LXNMR) was analyzed to yield a data set of 19 dipolar couplings. The presence of so many couplings, and in particular the dependence of some of them on the acetyl carbons enabled the investigation of the structure of the acetyl moiety and of possible cooperative motions about the aryl-carbonyl and carbonyl-methyl bonds. Methodological aspects, and approximations relating to the application of the vibrational correction procedure in the presence of large-amplitude torsional motions, are discussed. Results show that it is possible to discriminate between a continuous and a discrete conformer distribution about the angle phi(1) but not among a few proposed continuous shapes of U(iso)({phi}). In this study, the use of dipolar couplings with a non-negligible contribution from the indirect spin-spin coupling tensor J, (D(C8C9) in our case), for structural determination is extended from rigid to flexible molecules. The 1/2J(aniso)(C8C9) contribution was derived theoretically using the density functional theory linear response (DFT-LR) first-principles calculation of the J(C8C9) spin-spin coupling tensor.

The 5-Me of thyminyl (T) interaction with the neighboring nucleobases dictate the relative stability of isosequential DNA-RNA hybrid duplexes

Relative energetic contributions from the base-pairing [DeltaG(degrees)(bp)] vis-a-vis stacking [DeltaG(degrees)(stacking)] to the total free-energy of stabilization [DeltaG(degrees)(37)] for 14 pairs of isosequential hybrid DNA-RNA duplexes (taken from E. A. Lesnik and S. M. Freier, Biochemistry ,1995, 34, 10807) have been dissected in order to understand the differences in the intrinsic nature of the electrostatic forces that are responsible for the self-assembly of the heteroduplexes compared to homoduplexes. The pKa differences between the monomeric nucleotide 3'-ethylphosphates [(d/rN)pEt] as well as nucleotide 3',5'-bis-ethylphosphates [Etp(d/rN)pEt] in both 2'-deoxy (dN) and ribo (rN) series (N = A/G/C/T/U), as the model donor and acceptor (in which stacking is completely eliminated) mimicking those of the internucleotide monomer building blocks of a duplex, can be qualitatively used (P. Acharya, P. Cheruku, S. Chatterjee, S. Acharya and J. Chattopadhyaya, J. Am. Chem. Soc., 2004, 126, 2862) to understand the strength of base-pairing energies in different DNA-RNA (DR), RNA-DNA (RD), DNA-DNA (DD), and RNA-RNA (RR) duplexes. The study has led us to show the following. (1) As the number of excess %T in DR duplexes compared to the isosequential RD duplexes increase the differences in their thermal stabilization [DeltaT(m)](DR-RD) increase and vice-versa(2) The total relative stabilizations, [DeltaDeltaG(degrees)(37)](DR-RD) among the 14 pairs of isosequential DR and RD duplexes (E. A. Lesnik and S. M. Freier, Biochemistry ,1995, 34, 10807) are wholly dependent on the differences in the number of 5-Me(T) stacking interactions with the nearest-neighbors in the D strands of DR duplexes compared to that of the RD duplexes (3) In the relative stabilization of the DR or RD duplexes differences in the free-energy of stackings [DeltaDeltaG(degrees)(stacking)](DR-RD) play a more significant role than the differences in the free-energy of base-pairing, [DeltaDeltaG(degrees)(bp)](DR-RD). In contradistinction, our experimental data shows that RNA-RNA duplexes are more stable than DNA-DNA duplexes because of larger energy gain from the base-pairing in the former compared to the latter (P. Acharya, P. Cheruku, S. Chatterjee, S. Acharya and J. Chattopadhyaya, J. Am. Chem. Soc., 2004, 126, 2862).