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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3692164]

The Rydberg states in the vacuum ultraviolet photoabsorptionspectrum 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 energyUV 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 UVphotoelectron spectrum with an improved resolution yielded adiabatic and vertical ionizationenergies 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.

High quality photoelectron spectra of gaseous Fe(CO)5 excited by HeI, HeII, and ArI photons have been obtained. Major attention is focused on the primarily metal 3d ionizations, which occur in the binding energy region from 7 to 11 eV. Ionization to the 2E′ positive ion state (centered at 8.6 eV) clearly shows the effects of Jahn–Teller distortions in the positive ion. This ionization results in two ionization bands of approximately equal intensity and shape separated by 0.38 eV at room temperature. These bands broaden and the splitting increases to 0.47 eV at 473 °K. Ionization to the 2E″ positive ion state, centered at 9.9 eV, is much less influenced by the Jahn–Tellar effect. There is no discernable splitting of this band at room temperature. These observations are discussed in terms of the electronic structure and bonding of Fe(CO)5. Simple model calculations of the energies of the doubly degenerate electronic states in relation to the appropriate doubly degenerate normal vibrational modes are used to...

The He(I) and He(II) high resolution photoelectron spectra of osmocene are reported. Vibrational fine structure is observed in all the valence metal‐based ionizations and in the cyclopentadienyl π ionizations which derive from the e1g and e1u symmetry combinations. Analysis of the vibrational progressions found in the metal‐based ionizations provides a measure of the force constants and vibrational frequencies for the metal–ring stretch in the positive ions. The vibrational analysis for the 2E2(5/2) and 2E2(3/2) states of the osmocene cation [derived from the spin–orbit split ionization of the metal e2g(dx2−y2, dxy) set] indicates an 0.12 A greater metal–ring bond length in the cation compared to the neutral molecule. The sharp 2A1(1/2) ionization [correlating with removal of an electron from the metal a1g(dz 2) orbital] is observed to have a much shorter vibrational progression. The adiabatic ionization is the most intense (vertical) band of the series, indicating that there is no appreciable change in m...

The N(KVV) Auger spectra of gas phase pyridine (C5H5N) and borazine (B3N3H6), and of solid phase poly(2‐vinyl)pyridine (PVP) and hexagonal boron nitride [(BN)x] are reported and analyzed. The data indicate two Auger ‘‘fingerprint’’ types of nitrogen. Ammonia (NH3) is the prototype for the first, where three of the five valence electrons are σ bonding and the other two are the lone pair. This localized electronic structure gives rise to relatively sharp features in the N(KVV) spectrum. Typical of the second fingerprint type is pyridine, where there are two σ bonding electrons, a lone pair of electrons, and one electron contributing to the delocalized π system. Theoretical nitrogen Auger transition energies and intensities are calculated for pyridine to demonstrate the general origin of the overlapping features in the relatively broad N(KVV) spectrum of this molecule. PVP fits into the second fingerprint type while borazine and boron nitride give nitrogen Auger spectra more like ammonia. Approximate calculations using the equivalent core concept are used to clarify the relationship between the ammonia, borazine, and boron nitride spectra. It is shown that in these systems the initial Auger state (core–hole) largely localizes the bonds and lone pair on the nitrogen. The Auger spectra show that it is the σ, π and nonbonding orbital characters that provide the Auger fingerprint.

Gas phase core photoelectron spectroscopic (XPS) results are reported for a series of trimethylphosphine substituted molybdenum carbonyls: Mo(CO)6, Mo(CO)5(PMe3), cis‐Mo(CO)4(PMe3)2, trans‐Mo(CO)4(PMe3)2, and fac‐Mo(CO)3(PMe3)3. Core ligand additivity, defined as a constant shift in core ionizations with each successive step of ligand substitution, is indicated by these data. The shift per phosphine substitution is −0.65±0.10 eV for the molybdenum 3d5/2 ionization, −0.75±0.11 eV for the carbon (carbonyl) 1s ionization, and −0.78±0.09 eV for the oxygen 1s ionization. Comparison of core and valence data sets for these complexes illustrates a second principle, core–valence ionization correlation. The ratio of the Coulombic valence metal d level shifts to the core metal shifts is 0.74±0.06. This trend, in a system with extensively delocalized metal orbitals, shows that core and valence photoelectron spectroscopies are intimately related and that key additional understanding of electron distributions and bondi...

The ability of certain canonical orbitals of isolated molecular fragments to transfer largely unchanged to the molecular environment is examined. The separation of fragment canonical orbitals from the total molecular electronic problem is compared with the more familiar separations of atomic core orbitals and fragment localized orbitals. The specific example of the carbonyl functional group in formaldehyde is examined in detail. These studies lead to a new concept of valence electron only calculations in which the molecular valence electrons are assumed to move in an effective field provided by frozen molecular fragment canonical cores. In addition, for the case of assumed fragment canonical orbital transfer, perturbation theory analysis is found to be an efficient method of assessing the quality of the approximate wavefunction, thus eliminating much of the uncertainty as new systems are studied. The methods developed in the course of these studies offer certain practical advantages for the construction of approximate wavefunctions for large molecules. The details of application of these concepts to existing molecular orbtial methods are also presented.