The S-1/S-2 exciton interaction in 2-pyridone center dot 6-methyl-2-pyridone: Davydov splitting, vibronic coupling, and vibronic quenching

The excitonic splitting between the S-1 and S-2 electronic states of the doubly hydrogen-bonded dimer 2-pyridone center dot 6-methyl-2-pyridone (2PY center dot 6M2PY) is studied in a supersonic jet, applying two-color resonant two-photon ionization (2C-R2PI), UV-UV depletion, and dispersed fluorescence spectroscopies. In contrast to the C-2h symmetric (2-pyridone) 2 homodimer, in which the S-1 <- S-0 transition is symmetry-forbidden but the S-2 <- S-0 transition is allowed, the symmetry-breaking by the additional methyl group in 2PY center dot 6M2PY leads to the appearance of both the S-1 and S-2 origins, which are separated by Delta(exp) = 154 cm(-1). When combined with the separation of the S-1 <- S-0 excitations of 6M2PY and 2PY, which is delta = 102 cm(-1), one obtains an S-1/S-2 exciton coupling matrix element of V-AB, el = 57 cm(-1) in a Frenkel-Davydov exciton model. The vibronic couplings in the S-1/S-2 <- S-0 spectrum of 2PY center dot 6M2PY are treated by the Fulton-Gouterman single-mode model. We consider independent couplings to the intramolecular 6a' vibration and to the intermolecular sigma' stretch, and obtain a semi-quantitative fit to the observed spectrum. The dimensionless excitonic couplings are C(6a') = 0.15 and C(sigma') = 0.05, which places this dimer in the weak-coupling limit. However, the S-1/S-2 state exciton splittings Delta(calc) calculated by the configuration interaction singles method (CIS), time-dependent Hartree-Fock (TD-HF), and approximate second-order coupled-cluster method (CC2) are between 1100 and 1450 cm(-1), or seven to nine times larger than observed. These huge errors result from the neglect of the coupling to the optically active intra-and intermolecular vibrations of the dimer, which lead to vibronic quenching of the purely electronic excitonic splitting. For 2PY center dot 6M2PY the electronic splitting is quenched by a factor of similar to 30 (i.e., the vibronic quenching factor is Gamma(exp) = 0.035), which brings the calculated splittings into close agreement with the experimentally observed value. The 2C-R2PI and fluorescence spectra of the tautomeric species 2-hydroxypyridine center dot 6-methyl-2-pyridone (2HP center dot 6M2PY) are also observed and assigned. (C) 2011 American Institute of Physics.

N-H center dot center dot center dot pi hydrogen-bonding and large-amplitude tipping vibrations in jet-cooled pyrrole-benzene

The N-H center dot center dot center dot pi hydrogen bond is an important intermolecular interaction in many biological systems. We have investigated the infrared (IR) and ultraviolet (UV) spectra of the supersonic-jet cooled complex of pyrrole with benzene and benzene-d(6) (Pyr center dot Bz, Pyr center dot Bz-d(6)). DFT-D density functional, SCS-MP2 and SCS-CC2 calculations predict a T-shaped and (almost) C(s) symmetric structure with an N-H center dot center dot center dot pi hydrogen bond to the benzene ring. The pyrrole is tipped by omega(S(0)) = +/- 13 degrees relative to the surface normal of Bz. The N center dot center dot center dot ring distance is 3.13 angstrom. In the S(1) excited state, SCS-CC2 calculations predict an increased tipping angle omega(S(1)) = +/- 21 degrees. The IR depletion spectra support the T-shaped geometry: The NH stretch is redshifted by -59 cm(-1), relative to the "free" NH stretch of pyrrole at 3531 cm(-1), indicating a moderately strong N-H center dot center dot center dot pi interaction. The interaction is weaker than in the (Pyr)(2) dimer, where the NH donor shift is -87 cm(-1) [Dauster et al., Phys. Chem. Chem. Phys., 2008, 10, 2827]. The IR C-H stretch frequencies and intensities of the Bz subunit are very similar to those of the acceptor in the (Bz)(2) dimer, confirming that Bz acts as the acceptor. While the S(1) <- S(0) electronic origin of Bz is forbidden and is not observable in the gas-phase, the UV spectrum of Pyr center dot Bz in the same region exhibits a weak 0(0)(0) band that is red-shifted by 58 cm(-1) relative to that of Bz (38 086 cm(-1)). The origin appears due to symmetry-breaking of the p-electron system of Bz by the asymmetric pyrrole NH center dot center dot center dot pi hydrogen bond. This contrasts with (Bz)(2), which does not exhibit a 0(0)(0) band. The Bz moiety in Pyr center dot Bz exhibits a 6a(0)(1) band at 0(0)(0) + 518 cm(-1) that is about 20x more intense than the origin band. The symmetry breaking by the NH center dot center dot center dot pi hydrogen bond splits the degeneracy of the v(6)(e(2g)) vibration, giving rise to 6a' and 6b' sub-bands that are spaced by similar to 6 cm(-1). Both the 0(0)(0) and 6(0)(1) bands of Pyr center dot Bz carry a progression in the low-frequency (10 cm(-1)) excited-state tipping vibration omega', in agreement with the change of the omega tipping angle predicted by SCS-MP2 and SCS-CC2 calculations.

Vibronic Spectra of Jet-Cooled 2-Aminopurine center dot H2O Clusters Studied by UV Resonant Two-Photon Ionization Spectroscopy and Quantum Chemical Calculations

For understanding the major- and minor-groove hydration patterns of DNAs and RNAs, it is important to understand the local solvation of individual nucleobases at the molecular level. We have investigated the 2-aminopurine center dot H2O. monohydrate by two-color resonant two-photon ionization and UV/UV hole-burning spectroscopies, which reveal two isomers, denoted A and B. The electronic spectral shift delta nu of the S-1 <- S-0 transition relative to bare 9H-2-aminopurine (9H-2AP) is small for isomer A (-70 cm(-1)), while that of isomer B is much larger (delta nu = 889 cm(-1)). B3LYP geometry optimizations with the TZVP basis set predict four cluster isomers, of which three are doubly H-bonded, with H2O acting as an acceptor to a N-H or -NH2 group and as a donor to either of the pyrimidine N sites. The "sugar-edge" isomer A is calculated to be the most stable form with binding energy D-e = 56.4 kJ/mol. Isomers B and C are H-bonded between the -NH2 group and pyrimidine moieties and are 2.5 and 6.9 kJ/mol less stable, respectively. Time-dependent (TD) B3LYP/TZVP calculations predict the adiabatic energies of the lowest (1)pi pi* states of A and B in excellent agreement with the observed 0(0)(0) bands; also, the relative intensities of the A and B origin bands agree well with the calculated S-0 state relative energies. This allows unequivocal identification of the isomers. The R2PI spectra of 9H-2AP and of isomer A exhibit intense low-frequency out-of-plane overtone and combination bands, which is interpreted as a coupling of the optically excited (1)pi pi* state to the lower-lying (1)n pi* dark state. In contrast, these overtone and combination bands are much weaker for isomer B, implying that the (1)pi pi* state of B is planar and decoupled from the (1)n pi* state. These observations agree with the calculations, which predict the (1)n pi* above the (1)pi pi* state for isomer B but below the (1)pi pi* for both 9H-2AP and isomer A.

A longitudinal three-center study of craniofacial morphology at 6 and 12 years of age in patients with complete bilateral cleft lip and palate

In this longitudinal study, the craniofacial morphology and evaluated soft tissue profile changes, at 6 and 12 years of age in patients with complete bilateral cleft lip and palate (CBCLP) were compared. Lateral cephalograms from 148 patients with CBCLP, treated consecutively at three European cleft centers, Gothenburg (n (A) = 37), Nijmegen (n (B) = 26), and Oslo (n (C) = 85), were evaluated. Eighteen hard tissue and ten soft tissue landmarks were digitized. Paired t test, Pearson's correlation coefficients, and multiple regression models were applied for statistical analysis. ANOVA and Tukey-B, as a post hoc test, were used to evaluate the increments and compare centers. Hard and soft tissue data were superimposed using the generalized Procrustes analysis. For Nijmegen, the increments of the variables SNA, ANB, SN-NL, SN-ML, NL-ML, Snss, and Snpg were significantly different than the two other centers (p = 0.041 to <0.001). SNPg increments were significantly different between Nijmegen and Oslo (p = 0.002). The three cleft centers followed different treatment protocols, but the main differences in craniofacial morphology until 12 years of age were the growth pattern and the maxillary and upper incisor variables. Follow-up of these patients until facial growth has ceased, which may elucidate components for improving treatment outcome.