The spectroscopic and photophysical effects of the position of methyl substitution. I. 4- and 5-methylpyrimidine

Abstract

Laser-induced fluorescence excitation and dispersed fluorescence spectra of the first n-?r* transition of jet-cooled 2-methylpyrimidine have been recorded and analyzed. This work extends our earlier study of the spectroscopic and photophysical effects of methyl substitution in 4-and 5methylpyrimidine. An unusual Fermi resonance involving the 6~2: progression forms the focus of the present study. The 6~2: vibronic transition is observed to be split into a triad of transitions. Dispersed fluorescence spectra are used to identify the dark background state responsible for the Fermi resonance coupling as the 166 ' (3~;') vibration/internal rotation combination level. This level is selectively coupled by symmetry constraints to 6a' (0~;), leaving the 6~2' (le") level unperturbed. The positions and intensities of the triad of peaks in the excitation spectrum allow a quantitative determination of the 6a' (0~;)++16b ' (3a;') coupling matrix element of V = 4.1 cm-'. This vibration/internal rotation Fermi resonance is thus typical of the new types of routes to vibrational state mixing which are opened by methyl substitution. Higher members of the 6~2: progression are also involved in Fermi resonance mixing. However, in addition, these levels experience weaker, less state-specific coupling to a bath of same-symmetry states at that energy. The excitation spectrum provides an estimate of the average coupling matrix element of this second tier coupling of-1 cm-'.

S0 and S1 spectroscopy of jet cooled 9-cyano-10-methylanthracene: The methyl group as a molecular rotor

“…In effect, the ground state barrier extends over a relatively wide region, from $5 cm À1 for toluene [8] up to 800 cm À1 in 1-methyl-naphthalene [10]. The corresponding potential barrier for methyl group rotation in the electronically excited systems are as high as 740 cm À1 for 2-methylpyrimidine [14], and decreases down to only 23 and 6 cm À1 in the case of o-fluorotoluene [8] and tolunitrile in the S 1 state [7].…”

Electronic substituent and amino torsional coupling effects on intramolecular proton tunneling in 5-aminotropolone

“…In methyl groups attached to aromatic rings, several previous studies have shown that the onedimensional potential for internal rotation is often dominated by threefold terms even when only very small asymmetry exists in the positions adjacent to the methylated carbon atom. [35][36][37] The threefold barriers indicate that the methyl group sensitively distinguishes between a methyl hydrogen pointing toward the ring atom on one side versus the other. This same sensitivity may well carry over to the amino group's torsional potential.…”

Analytic First-Order Derivatives of Partially Contracted N-Electron Valence State Second-Order Perturbation Theory (PC-NEVPT2)

“…The 0-0 transition energy E 0−0 is defined as the difference between the ZPVE-corrected energies: E 0−0 = (E ES + E ES ZPVE ) − (E GS + E GS ZPVE ). In this study, 2-, 4-, and 5-methylpyrimidine (MP) molecules, for which experimental data 69,70 are available, were selected, and CASSCF, NEVPT2, XMS-CASPT2, and some linear-response time-dependent DFT (TD-DFT) 71 with B3LYP, CAM-B3LYP, and LC-BLYP calculations were performed. First, the structures were optimized using the VTZ basis set, and single-point energy calculations were performed using the AVTZ basis set.…”