NMR Relaxation Study of the Protonated Form of 1,8-Bis(dimethylamino)naphthalene in Isotropic Solution: Anisotropic Motion outside of Extreme Narrowing and Ultrafast Proton Transfer

Theoretical study of intramolecular hydrogen bond in selected symmetric "proton sponges" on the basis of DFT and CPMD methods

"Proton sponges," derivatives of prototypic 1,8-bis(dimethylamino)naphthalene (DMAN), exhibit remarkable basicity, which made them interesting for various experimental and theoretical studies. The details of bridged proton dynamics in protonated DMAN and its derivative denoted as TMGN (1,8-bis(tetramethylguanidino)naphthalene) were investigated on the basis of density functional theory (DFT) and Car-Parrinello molecular dynamics (CPMD) methods. Special attention was paid to the effects of symmetry of the molecular skeleton and the type of substituent on the bridged proton neighborhood statistics and dynamics. The metric parameter analyses of hydrogen bridge provided us with a conclusion that proton migration events in TMGNH⁺ are less numerous than in DMANH⁺, which can be rationalized by noticing the slower dynamics of large substituents of TMGN with respect to the smaller -N(Me)₂ groups of DMAN. The atomic velocity power spectra served as computational models of the vibrational signatures associated with the presence of the intramolecular hydrogen bond. A broad feature was registered for hydrogen bonds present in both compounds. The computations were verified by experimental data available.

First observation of ultrafast intramolecular proton transfer rate between electronic ground states in solution

Despite the importance of ultrafast (time scale exceeding 10(-11) s) intramolecular proton transfer (PT) events between electronic ground states in solution, experimental determination of the rates of such reactions has not yet been accomplished because of the limitations of the utilized methods. The objective of this study was to evaluate the PT rates of intramolecular O···H···O hydrogen-bonded systems in solution through the (1)H spin-lattice relaxation times of the hydroxyl protons, induced by the (1)H-(17)O dipolar interactions (T(1dd)(OH)), taking into account the contribution of the OH reorientational motion to T(1dd)(OH). Solutions of the benzoic acid dimer (BA dimer), 1-benzoyl-6-hydroxy-6-phenylfulvene (Fulvene), and dibenzoylmethane (DBM) were chosen as test systems. For Fulvene in CCl(4), the PT time, τ(PT), was deduced to be 7 × 10(-11) s. In the case of the BA dimer in CCl(4), the τ(PT) value was considerably greater than the OH reorientational correlation time, τ(R(OH)) = 4.3 × 10(-11) s. In contrast, the experimental results for DBM in CCl(4) indicated that the proton is located about midway between the two oxygen atoms, that is, the PT potential energy surface is a single well or a double well with a PT barrier near or below the zero-point energy.

Determination of the rotational diffusion tensor of porphycene by 13C and 1H spin relaxation methods

The longitudinal (13)C and (1)H relaxation rates were determined in porphycene in CD(2)Cl(2) solution. These data, augmented by (13)C{(1)H} NOE enhancements were numerically analyzed to evaluate the rotational diffusion tensor of the molecule and the vibrational correction for the one-bond (13)C-(1)H dipolar couplings. The (13)C and (1)H relaxation data seem to be consistent with each other, and the emerging picture of the rotational dynamics of porphycene compares well with the results that can be found in the literature.

Nuclear-spin relaxation in nonrigid molecules: discrete multisite local dynamics combined with anisotropic molecular reorientation

Nuclear-spin relaxation is considered in a molecular system undergoing two types of dynamic processes: asymmetric-top small-step rotational diffusion and discrete multisite local jumps. The two processes are assumed to be uncorrelated. Time correlation functions for relevant rank-two interactions and corresponding spectral density functions are derived for a general relation between the characteristic rate constants. In addition, limiting cases of fast and slow local motions and of some specific jump conditions are also investigated.