Rotationally Resolved S1− S0 Electronic Spectra of 2, 6-Diaminopyridine: A Four-Fold Barrier Problem (original) (raw)
Related papers
2010
High resolution gas phase electronic spectra were recorded for 2,6-diaminopyridine (26DAP) and pyrrolidinobenzonitrile (PYRBN). A comparison of the electronic properties of the nitrogencontaining rings aniline, 2-aminopyridine, and 2,6-diaminopyridine (26DAP) shows that the potential energy surface of the molecule is significantly affected as more nitrogen atoms are added to the system. High resolution, rotationally resolved spectra of four vibrational bands in the S 1-S 0 electronic transition of 26DAP were obtained in order to explain these changes. The zigzagging inertial defects point to a double minimum excited state potential energy surface along the coupled amino group inversion vibrational mode, which becomes a four-fold well (and barrier) problem when the existence of two nearly degenerate isomers is taken into account. Assuming that the molecules are in the lower energy, opposite-side configuration, ab initio calculations were performed using the MP2/6-31G** level of theory to create a potential energy surface modeling the simultaneous antisymmetric NH 2-inversion mode. The calculated potential energy surface shows a ground electronic state barrier to simultaneous NH 2 inversion of ~ 220 cm-1 , and a fit to experimental vibrational energy level spacings and relative intensities v produces an excited electronic state barrier of ~ 400 cm-1. The ground state barrier is less than that in aniline, but the excited state barrier is larger. Pyrrolidinobenzonitrile (PYRBN), a derivative of DMABN, has been examined here using high resolution fluorescence excitation spectroscopy in the presence of an electric field varying from 0-846 V/cm. The b-type fluorescence band reveals that the transition moment is along the short, in plane axis of the molecule. Upon excitation the inertial defect remains unchanged, which suggests that the molecule's planarity remains constant. The dipole moment is found to increase from 8.06 to 10.45 D upon electronic excitation. This analysis of PYRBN leads to many interesting comparisons to 1PP including their transition moments, dipole moments, and
Chemical Physics Letters, 2000
An effective one-dimensional Hamiltonian has been determined for the NH inversion motion in aniline. The anharmonic 2 potential is represented by a quartic polynomial, fitted to the experimental vibrational transitions in the electronic states S 0 and S . In the S state the barrier height is about 580 cm y1 and the equilibrium angle between the NH and the ring planes 1 0 2 is 448, while in the S state the potential well is extremely flat without a well-defined equilibrium angle. These results are 1 compared with those obtained by ab initio methods. The long-standing discrepancy between the rotational and vibrational results is partially clarified. q
The Journal of Physical Chemistry, 1996
The geometrical and electronic structures, the force constants, the vibrational frequencies, and the potential energy distributions for various isotopomers of 4,4′-bipyridine (44BPY) in the ground state S 0 and for the corresponding anion radical 44BPY •and N,N′-dihydro cation radical 44BPYH 2 •+ have been computed by the 3-21G(+ *) ab initio method, with the RHF formalism for the ground state S 0 and with the ROHF formalism for the ionic species. The theoretical results have been compared to the available vibrational data. The very good agreement between calculations and experiment has allowed the determination of the conformations of the S 0 state and of the ion radicals. The ground state is characterized by an aromatic structure in which the two pyridyl rings are linked in a twisted conformation, and the ionic species present a quinoidal planar distortion, more marked in the cation radical than in the anion radical.
Zeitschrift für Naturforschung, 2010
The molecular structures, vibrational frequencies, and corresponding vibrational assignments of 2-amino-3-, 4-, and 5-nitropyridine have been calculated by using ab initio Hartree-Fock (HF) and density functional theory (B3LYP) methods with 6-311++G(d,p) basis set level. The calculated vibrational frequencies and optimized geometric parameters (bond lengths and bond angles) were found to be in well agreement with the experimental data. The comparison of the observed and the calculated results showed that the scaled B3LYP method is superior to the scaled HF method for both the vibrational frequencies and the geometric parameters. For well fitting the calculated and the experimental frequencies we used scale factors obtained from the ratio of the frequency values of the strongest peaks in the calculated and the experimental spectra. These obtained scales seem to cause the better agreement of the gained vibrations to the experimental data.
Vibrational spectroscopic study of 4-aminopyrimidine complexes
Journal of Molecular Structure, 2009
a b s t r a c t A series of Hofmann-type 4-aminopyrimidine complexes with the general formula M(4APM) 2 M 0 (CN) 4 {where M = Mn, Zn or Cd; M 0 = Pd or Pt; 4APM = 4-aminopyrimidine} have been prepared for the first time and their FT-IR (4000-400 cm À1 ) and FT-Raman (4000-50 cm À1 ) spectra measured. 4APM is coordinated to M(II) through one of the pyrimidine ring nitrogen atoms as a monodentate ligand; the exo-cyclic nitrogen is not involved in the complex formation. Vibrational bands arising from both 4APM and the M 0 (CN) 4 group were assigned. Comparison of the low frequency vibrational wavenumbers of the tetracyanometallate sheets of the isostructural compounds allow a tentative assignment for m(M 0 AC), d(M 0 C"N), p(M 0 C"N), d(CM 0 C), m(MAN) 4APM , m(MANC) and d(NMN) 4APM vibrations. The coordination effect on 4APM vibrational wavenumbers is analysed.
Vibrational spectra and normal co-ordinate analysis of 2-aminopyridine and 2-amino picoline
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2006
The observed gas-phase IR Frequencies for Forty-four fundamentals of methyl thionitrite (CH,SNO) and its d,-, '"C-, and "N-substituted analogs have been used to calculate a nineteen-parameter symmetry valence force field The final refinement resulted in an average error of less than 4 cm-' ( -0.5%) between the calculated and observed frequencies for the four isotopomers. Contrary to earlier reports, relative intensities, isotopic frequency shifts, as well as the calculated potential-energy distribution, all support the assignment of u(CS) to a higher frequency than that of u(SN). For the normal molecule,
Journal of the Chinese Chemical Society, 2003
Three harmonic potential‐energy surfaces of 1A1, 1B1, and 1B2 singlet states of pyridine have been obtained by using ab initio method. Geometric properties and force fields of these states have been determined with the complete‐active‐space self‐consistent‐field (CASSCF) theoretical method. Vibrational frequencies of two excited states have been assigned on the basis of the potential energy distribution from normal mode analysis. The resulting properties are used to simulate the experimental absorption spectrum. From the spectral simulation the totally symmetric vibrations with large Huang‐Rhys factors have been identified as ν12 and ν6a modes in the 1B1 state; ν1 and ν12 modes in the 1B2 state, indicating that these modes have strong vibronic coupling between ground and excited states.
Der Pharma Chemica
Present work deals with the quantum mechanical study of the molecular structure of two wellknown voltage sensitive potassium channel blockers: 4, Aminopyridine and 3, 4 Diaminopyridine. The equilibrium geometry, harmonic vibrational frequencies and HOMO-LUMO gap have been calculated by the density functional theory (DFT), employing 6-311++G (2d, 2p) as the basis set. A detailed interpretation of the infrared and Raman spectra of 4, aminopyridine and 3, 4 Diaminopyridine in terms of the potential energy distribution (P.E.D) is reported. The similarities and differences between the vibrational spectra of the two molecules studied have been highlighted. The thermodynamic calculations related to the title compounds were also performed at B3LYP/6-311++G (2d, 2p) level of theory.
The Journal of Chemical Physics, 2001
High resolution, vibrationally resolved, near-edge x-ray absorption fine structure ͑NEXAFS͒ spectra at the C 1s and N 1s ionization thresholds of pyridine and deuterated d 5-pyridine in the gas phase have been recorded. The high resolution of 65 meV ͑150 meV͒ at the C s ͑N 1s͒ ionization thresholds reveals vibrational structures in the spectra. Detailed ab initio and density functional theory ͑DFT͒ calculations were performed to interpret the experimental spectra and to assign the observed peaks. In particular we focused on the previously unexplained intensity ratio for the two components of the C 1s→1* transition. For this transition the vibrational structure is included through a linear coupling model in the DFT calculations and leads to the experimentally observed ϳ2:3 intensity ratio between the two * components in the C 1s spectrum rather than the ϳ3:2 ratio obtained without vibrational effects. After inclusion of relaxation effects in the excited states, in addition to the vibrational effects, both theoretical methods yield almost perfect agreement with experiment.