The electronic structure of some heterocycles with bridgehead nitrogen: photoelectron spectra and ab initio molecular orbital calculations

He(I) and He(II) photoelectron spectra are reported for the cycl[3,3,3]azine (1), cycl[3,2,2]azine (2), indolizine (6) and imidazo[1,2-a] pyridine (7), as well as He(I) spectra for related compounds (3–5). Ab initio molecular orbital calculations have been used to assign the spectra of 1, 2, 3, 6 and 7, and to give information about the nature of the π-electron energy levels. The first IP for 1 is singularly low (5.86 eV), and this has been interpreted in terms of occupancy of the 1a₁'' orbital which is normally vacant in related compounds. In the cyclazines, the nitrogen lone pair seems to be split into two π-levels.     

1,5-Diamino-1H-1,2,3,4-tetrazolium picrate: X-ray molecular and crystal structures and ab initio MO calculations

The crystal and molecular structures of 1,5-diamino-1H-1,2,3,4-tetrazolium picrate (DATP) were determined by X-ray diffraction analysis. The tetrazolium cation in DATP has a structure with protonated N4 atom of the ring. Two amino groups in the cation are found to be rather different. The 5-amino group lies in the plane of the tetrazole ring and valence angles around the N atom are close to 120°, which indicates sp² hybridization of atomic orbital of the nitrogen atom. In contrast, valence angles around the N atom of the 1-amino group are close to tetrahedral angle, which suggests sp³ hybridization. The exocyclic C-N bond in the cation is substantially shorter than that in 1,5-diaminotetrazole. The obtained results indicate a conjugation between the π-system of the tetrazole ring and the 5-amino group. The results of ab initio calculations of electronic structure and relative stability for various tautomeric forms of protonated 1,5-diaminotetrazole using MP2/6-31G* and B3LYP/6-31G* levels of theory are in a good agreement with X-ray data and show that there are differences in σ-electron overlap populations for the C-N bonds in the cation in DATP, while π-electrons are delocalized.     

Rydberg or Valence? The Long‐Standing Question in the UV Absorption Spectrum of 1,1′‐Bicyclohexylidene

The electronic excited states of the olefin 1,1′‐bicylohexylidene (BCH) are investigated using multiconfigurational complete active space self‐consistent‐field second order perturbation theory in its multi‐state version (MS‐CASPT2). Our calculations undoubtedly show that the bulk of the intensity of the two unusually intense bands of the UV absorption of BCH measured with maxima at 5.95 eV and 6.82 eV in the vapor phase are due to a single ππ* valence excitation. Sharp peaks reported in the vicinity of the low‐energy feature in the gas phase correspond to the beginning of the π3s_R Rydberg series. By locating the origin of the ππ* band at 5.63 eV, the intensity and broadening of the observed bands and their presence in solid phase is explained as the vibrational structure of the valence ππ* transition, which underlies the Rydberg manifold as a quasi‐continuum.     

π-Orbital electron–energy contributions to chemical reaction heats,I. Reactions involving only linear molecules containing up to three first–row atoms

Ab initio calculations of various expectation energies have been made for the reactant and product species in six reactions that involve only small linear molecules. The reactions include fission by hydrogen, addition of hydrogen, exchange of triply bonded atoms, fluorination, and oxygen atom transfer. The change in total electronic energy is not invariably the result of changes in inner shell energy and outer shell σ- and π-electron energies simply augmenting each other, but in several cases there is a complex interplay of opposing effects. This approach gives a different insight into the energetic aspects of changes in bonding from that derived from the concept of shared electron pairs in σ and π bonds together with lone pairs in valence shells. Changes in π-electron energy are shown to be important in a reaction in which neither reactant nor product molecules contain π bonds in the usual chemical sense. While in a reaction in which there is a complete change in the nature of the triple bonds, and hence the π bonding, the change in π-electron energy makes a smaller contribution than either the change in inner shell or the outer shell σ-electron energies.     

The electronic structure of aromatic molecules: ab inito molecular orbital studies and photoelectron spectra for the aza-derivatives of indole,benzofuran and benzothiophen

The He(I) photoelectron spectra for the aza-derivatives of benzofuran (4 and 7), benzothiophen (8) and indole (3a, 3b, 6a, 9b), and combined He(I) and He(II) spectra of the 1,2-benzisothiazole (5), 1,2,3-benzothiadiazole (11) and benzotriazole (9a) have been obtained and assigned by a combination of heteroatom substituent effects and ab initio molecular orbital calculations. The variations in lone pair levels (LP_NLP_O, LP_S) and π-levels, between these and the monocyclic compounds are discussed.     

All-valence-electron Cl calculations on the electronic spectrum of diborane

All-valence-electron Cl calculations have been carried out for diborane B₂H₆ and its positive ion employing a rather large double-zeta AO basis including polarization functions in order to study the electronic spectrum of this system. Transitions from four different valence MOs are found to lead to low-lying electronic transitions of both Rydberg and valence type in each case. Ad mixture of valence character in the otherwise Rydberg-like (nx, 3s), (ny, 3s) and (σ, 3pz) transitions calculated to lie between 11.0 and 11.6 eV is indicated as being primarily responsible for the highly intense shoulder found in this region of the B₂H₆ spectrum. The other strong feature with essentially continuous absorption peaking at 9.3 eV is suggested to result from superposition of several Rydberg-type transitions in the generally broad absorption pattern expected for the ¹(π,π^*) species at significantly higher vertical excitation energy. Quite good agreement is obtained between calculation and experiment for all of the six lowest IPs of diborane and also for the locations of the ¹(n, π^*) and ¹(σ, π^*) transitions previously assigned to the two weak features observed at 6.8 and 8.3 eV in this spectrum.     

Multiconfigurational perturbation theory (CASPT2) applied to the study of the low-lying singlet and triplet excited states of cyclopropene

The electronic spectrum of cyclopropene has been studied using multiconfigurational second-order perturbation theory (CASPT2) with extended ANO-type basis sets. The calculation comprises two valence states and the 3s, 3p, 3d members of the Rydberg series converging to the π and σ ionization limits. A total of twenty singlet and twenty triplet excited states have been analyzed. The results confirm the valence nature of the lowest energy singlet-singlet band and yield a conclusive assignment: the first dipole-allowed transition in cyclcopropene is due to absorption to a (σ → π*) state. The (π → π*) (V) state is interleaved among a number of Rydberg states in the most intense band of the system. The remaining spectral bands are due to Rydberg transitions of higher energy. The two lowest singlet-triplet transitions involve the same valence states. The results are in agreement with available experimental data and provide a number of new assignments of the experimental spectra.     

Through-space interaction in non-conjugated acyclic dienes studied by photoelectron spectroscopy

The He (I) PE-spectra of the non-conjugated acyclic dienes 1 (n), n = 5, 6, 7, 8, 9 are reported. The π-level splitting in the molecules amounts respectively to 0·34, 0·46 (or 0·31), 0·41, 0·21 and 0·14 eV. Arguments based on the observed fine structure of the first two bands suggest there is no crossing of the π-levels, and EHT calculations on different conformations of 1 (5) and 1 (6), as well as comparisons with other available data, indicate a through-space dominated interaction throughout the series 1 (n).     

A note on the importance of d orbitals in the calculation of effective interactions and the excitation spectra of atoms and molecules

The focal point of our discussion is the examination of truncated basis sets used in obtaining an accurate first principles clculation of the effective valence shell Hamiltonian by the canonical transformation-cluster expansion approasch. Subsequent diagonalization of this effecitve valence shell hamiltonian yields the valence shell transition energies. A detailed analysis of numerical results obtained using a number of different basis sets of hydrogen-like orbitals together with rigorous symmetry arguments celarly demonstrates the special role played by d orbitals in computing the ³P → ¹D transition energy in carbon. The failure of early attempts to calculate the effective Hamiltonian for ethylene from first principles is examined in the light of recent ab initio calculations on ethylene involving d orbitals and the computations reported in this paper. We conclude that accurate calculations of the effective valence shell Hamiltonian for molecules must consider d orbitals in the excited orbital basis set.     

Schwingungsverhalten von dimethylsulfid,dimethyl-sulfoxid,dimethylsulfon und den entsprechenden per-deuterierten verbindungen: 2. Mitt. normalkoordinatenbehandlung

Normal coordinate calculations on dimethyl sulphide, dimethyl sulphoxide, dimethyl sulphone and their perdeuterated derivatives have been carried out. The GF-matrix method of Wilson was applied using the general harmonic valence force field. The calculated frequencies of the normal vibrations agree satisfactorily with the corresponding values obtained from the spectra. In addition the potential energy distributions were calculated and discussed. It has been found that the potential energy of the symmetric SO valence vibration of dimethyl sulphone is located only partly in the SO bond, like in dimethyl sulphoxide.     

A study of the electronic structure of phenylsilanes by X-ray emission spectroscopy and quantum chemical calculation methods

The electronic structure of a series of phenylsilanes Ph_4?n SiH _n (n = 0?C3) is studied by X-ray emission spectroscopy and quantum chemical calculations by the density functional theory method. Based on the calculations theoretical X-ray emission SiK??₁ spectra of phenylsilanes Ph_4?n SiH _n (n = 0?C4) are constructed and their energy structure and shape turn out to be well consistent with experiment. The distribution of the electron density of states with different symmetry of Si, C, H atoms are also constructed. An analysis of the obtained X-ray fluorescent SiK??₁ spectra and the distribution of the electron density of states in Ph₄Si and Ph₃SiH compounds shows that their energy structure is mainly determined by a system of the energy levels of phenyl ligands weakly perturbed by interactions with valence AOs of silicon. In the energy structure of MOs of the PhSiH₃ compound, energy orbitals related to t ₂ and a ₁ levels of tetrahedral SiH₄ are mainly presented.     

Beiträge zur Chemie der Pyrrolpigmente, 2. Mitt.: Die Röntgenphotoelektronenspektren einiger Pyrrolpigmente

The charge densities on the nitrogen atoms of symmetrically substituted dipyrromethenes, porphins, chlorins and bacteriochlorins were determined by X-ray photoelectron spectroscopy of the N_1s-levels. Calculation by means of a π-electron- and an all-valence-electron-method (SCF-LCAO-MO-PPP and CNDO/2) reveals good correlation of both methods with experiment. Both are comparable in predicting the energy of the highest occupied orbitals which were studied by measuring the valenceband regions.     

Electron diffraction study of the thionyl fluoride molecular structure

The electron diffraction study of thionyl fluoride yielded the following geometrical parameters (r_a structure): S-O 1.420±0.003 Å, S-F 1.583±0.003Å, O-S-F 106.2±0.2° and F-S-F 92.2±0.3°. The average structure (r_α°) is also given. Some of the variations in the molecular geometries of SOX₂ and SO₂X₂ molecules (X = F or Cl) involving the valence shell electron pair repulsion theory are discussed.     

MO-LCAO Calculations of Polymethines

Idealized polymethines are defined as conjugated chain molecules with complete bond equilization and charge alternation. A constant π-bond order along the chain with N atoms has been derived for three types of polymethines which differ by the nature of the heteroatomic end groups. Corresponding to the number of π-electrons they are termined (N + 1)π-, Nπ- or (N ? 1)π-polymethines. This classification embraces closed shell as well as open shell systems. The relations for the energy eigenvalues, coefficients and derived quantities have been obtained in closed form. The results for the different classes of polymethines are strongly related. Their molecular properties should be similar.     

The oxygen Kα x-ray emission spectra of fluorinated anisoles and pentafluorophenol

Oxygen and fluorine K_α X-ray emission spectra have been obtained for a number of oxygen-containing compounds: H₂O, CH₃OH, C₆H₅OH, C₆H₅OCH₃, C₆F₅OH and 4-XC₆F₄OCH₃ (X = F, OCH₃, CF₃) in the solid or gaseous states and interpreted on the basis of the UV photoelectron and ESCA data and the results of MINDO/3 calculations. The mixing of the oxygen 2pAO with the highest occupied π-orbitals of the benzene ring is concluded to be small. The main contribution of the 2p(O)AO is shown to be to the system of σ-levels and lower-lying π-levels. CH₃OH is assumed to have hyperconjugation. Comparison of the electronic structures of oxygen in phenol and anisole with those in their polyfluorinated analogues shows the reduced effectiveness of oxygen 2pAO conjugation with the π-system of the benzene ring in the latter cases.     

The shell structure of3He droplets

Liquid³He droplets and their shell structure are investigated in the framework of mean-field calculations using a recently developed effective energy functional which employs realistic finite range interactions throughout. The samples range fromN=20 up toN=1360. The shell closures found agree with the harmonic oscillator scheme up toN=168. Above that, magic droplets appear atN=198, 274, 300, 368, 398, 482, 516... The first and second differences of the free energy are computed as signals for the shell structure in future experiments. Shell closures are found to be still visible in these signals at comparatively high temperatures. The finite range character of the effective interaction generates oscillations in the density profile which are suppressed in comparable calculations with zero range interactions.     

A study of valence ionization of ammonia with synchrotron radiation

Angular distributions of photoelectrons from the three valence levels of molecular ammonia, 2a₁ with a binding energy (BE) of 27.7 eV, 1e (BE = 16.3 eV) and 3a₁ (BE = 10.9 eV) have been measured in the 30–130 eV photon energy range. Comparison is made in the case of the 3a₁ molecular orbital with the calculated atomic N 2p asymmetry parameters. Photoionization branching ratios have also been obtained in this photon energy range and good agreement has been found with previous quasiphotoionization measurements and with the one-center-expansion calculations of Cacelli et al. The satellite structure in the inner valence (2a₁) region has been investigated at 60 eV and compared with a number of energy and intensity calculations.     

Substituent effects in second-row molecules: Basis set performance in calculations of normal valency phosphorus and sulfur compounds

Ab inito molecular orbital calculations of the phosphorus- and sulfur-containing series PH₂X, PH₃X⁺, SHX, and SH₂X⁺ (X = H, CH₃, NH₂, OH, F) have been carried out over a range of Gaussian basis sets and the results (optimized geometrical structures, relative energies, and electron distributions) critically compared. As in first-row molecules there are large discrepancies between substituent interaction energies at different basis set levels, particularly in electron-rich molecules; use of basis sets lower than the supplemented 6-31G basis incurs the risk of obtaining substituent stabilizations with large errors, including the wrong sign. Only a small part of the discrepancies is accounted for by structural differences between the optimized geometries. Supplementation of low level basis sets by d functions frequently leads to exaggerated stabilization energies for π-donor substituents. Poor performance also results from the use of split valence basis sets in which the valence shell electron density is too heavily concentrated in diffuse component of the valence shell functions, again likely to occur in electron-rich molecules. Isodesmic reaction energies are much less sensitive to basis set variation, but d function supplementation is necessary to achieve reliable results, suggesting a marginal valence role for d functions, not merely polarization of the bonding density. Optimized molecular geometries are relatively insensitive to basis set and electron population analysis data, for better-than-minimal bases, are uniform to an unexpected degree.