Search

The first vacuum ultraviolet absorption spectrum of a 1,2,4-triazole has been obtained and analyzed in detail, with assistance from both an enhanced UV photoelectron spectroscopic study and ab initio multi-reference multi-root configuration interaction procedures. For both 1H- and 1-methyl-1,2,4-triazoles, the first ionization energy bands show complex vibrational structure on the low-energy edges of otherwise unstructured bands. Detailed analysis of these bands confirms the presence of three ionized states. The 6-7 eV VUV spectral region shows an unusual absorption plateau, which is interpreted in terms of the near degeneracy of the first two ionization energies, leading to a pseudo Jahn-Teller effect. The 'fingerprint' of the ionization spectrum yields band origins for several Rydberg states. The configuration interaction study shows that although the equilibrium structure for the first cation is effectively planar, the second cation shows significant twisting of the ring system. Some calculated singlet electronic states also show skeletal twisting in which the ring C-H is substantially out of plane. [ABSTRACT FROM AUTHOR]

The Rydberg states in the vacuum ultraviolet photoabsorption spectrum of 1,2,3-triazole have been measured and analyzed with the aid of comparison to the UV valence photoelectron ionizations and the results of ab initio configuration interaction (CI) calculations. Calculated electronic ionization and excitation energies for singlet, triplet valence, and Rydberg states were obtained using multireference multiroot CI procedures with an aug-cc-pVTZ [5s3p3d1f] basis set and a set of Rydberg [4s3p3d3f] functions. Adiabatic excitation energies obtained for several electronic states using coupled-cluster (singles, doubles, and triples) and complete active space self-consistent field procedures agree well with experimental values. Variations in bond lengths with the electronic state are discussed. The lowest energy UV band (∼5.5-6.5 eV) is assigned to three electronically excited states and demonstrates the occurrence of a nonplanar upper state on the low energy side. A UV photoelectron spectrum with an improved resolution yielded adiabatic and vertical ionization energies and reorganization energies for several of the lowest cationic states. As well as excitations to the s, p, d-Rydberg states are the excitations consistent with an f-series. [ABSTRACT FROM AUTHOR]

Two possible conformers of the dinuclear complex Mn Re(CO)10, each of C4v symmetry, with eclipsed and staggered conformations, have been analyzed theoretically. Using both the B3LYP and BP86 density functionals we find that the staggered form is lower in energy. A determination of the B3LYP potential energy surface as a function of the Mn-Re distance is presented for both conformers. The computed bond lengths, bond angles, and rotational constant for the staggered conformation compare favorably with the results from microwave experiments. The harmonic frequencies for the staggered structure have been determined using several basis sets, with both analytical and finite difference methods. These unscaled vibrational frequencies, together with their intensities for both infrared and Raman activity, are used to assign the three most intense experimental IR and Raman bands, and in particular, the νCO region. The lowest A2 vibration was calculated to occur at 41 cm-1 in the staggered conformer; this frequency becomes imaginary in the (saddle point) eclipsed form. Several fundamentals remain to be observed experimentally. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Two possible conformers of the dinuclear complex Mn Re(CO)10, each of C4v symmetry, with eclipsed and staggered conformations, have been analyzed theoretically. Using both the B3LYP and BP86 density functionals we find that the staggered form is lower in energy. A determination of the B3LYP potential energy surface as a function of the Mn-Re distance is presented for both conformers. The computed bond lengths, bond angles, and rotational constant for the staggered conformation compare favorably with the results from microwave experiments. The harmonic frequencies for the staggered structure have been determined using several basis sets, with both analytical and finite difference methods. These unscaled vibrational frequencies, together with their intensities for both infrared and Raman activity, are used to assign the three most intense experimental IR and Raman bands, and in particular, the νCO region. The lowest A2 vibration was calculated to occur at 41 cm-1 in the staggered conformer; this frequency becomes imaginary in the (saddle point) eclipsed form. Several fundamentals remain to be observed experimentally. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The first rotational spectrum of a dinuclear complex, MnRe(CO)[sub 10], has been obtained using a high-resolution pulsed beam microwave spectrometer. Sixty-four hyperfine components of the J=11→J[sup ′]=12 and J=12→J[sup ′]=13 rotational transitions were measured for two rhenium isotopomers. The B values obtained from the experiment are B=200.36871(18) MHz for the [sup 187]Re isotopomer and B=200.5561(10) MHz for the [sup 185]Re isotopomer. The measured rotational constants are in reasonably good agreement with the B values calculated from the x-ray diffraction structural data, and from theoretical calculations. The gas-phase Mn–Re bond distance is approximately 2.99 Å, and the calculated value is only slightly longer. The experimental quadrupole coupling constant for the manganese atom is eQq[sub aa] ([sup 55]Mn)=-16.52(5) MHz, and the corresponding quadrupole coupling constants for the two rhenium isotopomers are eQq[sub aa] ([sup 187]Re)=370.4(4) MHz and eQq[sub aa] ([sup 185]Re)=390.9(6) MHz. The quadrupole coupling constants were also determined from a variety of theoretical calculations, with very large Gaussian orbital bases. The best estimates, at a nonrelativistic level, are eQq[sub aa] ([sup 55]Mn)=0.68 MHz and eQq[sub aa] ([sup 187]Re)=327.6 MHz with a 874 GTO basis set, but the results are very basis set dependent, especially the sign of the Mn quadrupole coupling. Very slight bending of angles MnC[sub eq]O[sub eq] and ReC[sub eq]O[sub eq] angles is found in the calculations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Fe2 molecule is a typical transition metal dimer which has a rather large dissociation energy and a small bond distance compared with the inter-nuclear distance in the crystalline metal. We have investigated the Fe2 molecule with multireference self-consistent-field (MCSCF) and multireference configuration interaction (CI) calculations. The dissociation energy (De), the equilibrium nuclear distance (Re), and the zero-point frequency (ωe) were calculated (with observed in parentheses) as 1.57 (1.30±0.22) eV, 2.06 (1.87 to 2.02) Å, and 260.9 (299.6) cm-1, respectively. Thus the agreement between experiment and calculation is very satisfactory, and is a marked improvement on previous theoretical studies. The contribution of the d electrons to the bonding is important and a proper description of correlation effects among the d electrons is indispensable. [ABSTRACT FROM AUTHOR]

The excitation and ionization spectra of TiCl4 have been studied theoretically by the symmetry adapted cluster (SAC)/SAC-CI and multireference CI (MRD-CI) methods. The calculated spectra show good agreement with the observed spectra. The present results indicate several new assignments for the excitation spectrum. The peaks below 8.0 eV are assigned to valence excitations and those at 9.35 and 10.04 eV are assigned to Rydberg-type excitations within chlorine ligands. The ordering of the ionized states in the outer valence region is (1t1)-1 < (3t2)-1 < (1e)-1 < (2t2)-1 < (2a1)-1, which supports the result of earlier Green’s function calculations. [ABSTRACT FROM AUTHOR]