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The formation of Pd2N2 from the cocondensation of effusive beams of Pd and N2 in neon and argon matrices is evidenced by absorptions in the range of 2200–1800 cm-1. In argon, selective irradiation in the near-infrared and visible ranges leads to interconversions between three structures, distinguished by the stretching frequency of the diatomic N2: Bridged T-shaped (νNN at 1990 cm-1), side on (νNN at 2178 cm-1) and parallel (νNN at 1823 cm-1). For the first two structures, the νNN mode is also accompanied by another signal below 500 cm-1. An extra feature close to 490 cm-1, not sensitive to irradiation at λirr>400 nm and also assignable to a molecule with the same Pd:N2 stoichiometry (2:1), corresponds to a centrosymmetrical Pd–N–N–Pd structure with an inactive νNN mode close to 2141 cm-1, as deduced from the observation of a weak signal close to 2630 cm-1 associated with this species and assignable to the combination νNN+νPdN. All these experimental data and their structural implications are fully supported by theoretical calculations [density functional theory (DFT)]. On the basis of this comparative study, we have obtained a reliable theoretical description of the spectroscopic data using the metageneralized gradient approximation functional within the unrestricted DFT (UDFT) formalism for all spin multiplets. We have also searched a stable electronic solution for each multiplet (particularly for the singlet state), in order to account for the nondynamic correlations. [ABSTRACT FROM AUTHOR]

For the first time the coexistence of a σ- and a π-complex in the C4H4O:HCl system has been observed, in the same supersonic expansion of a molecular jet seeded with argon (or helium) or in a flow-cooled cell at 240 K. This is an exception to the third of the Legon–Miller rules which claims the σ-structure to be the only one to exist. On the grounds of energetic considerations and band contour simulations, two observed bands at 2787.7 and 2795.5 cm-1 of the νs HCl stretching frequency are assigned to the two complexes, recorded as Fourier transform infrared spectra with a resolution between 0.2 and 0.5 cm-1. Complementary calculations show that the use of the standard second-order Mo\ller–Plesset perturbation theory may be erroneous for such a complex, due of the overestimation of the dispersion contribution with respect to the electrostatic term. It is finally established that only a balanced version of the second-order Mo\ller–Plesset perturbation method, spin-component scaled-MP2, or a higher level of theory like a coupled-cluster approach, can provide a reliable energetic analysis for this complex. [ABSTRACT FROM AUTHOR]

Three gas-phase diatomic trications Se23+, Te23+, and LaF3+ have been produced by Ar+ ion beam sputtering of Se, Te, and LaF3 surfaces, respectively. These exotic molecular ions were detected at noninteger m/z values in a magnetic sector mass spectrometer for ion flight times of >=13 μs that correspond to lower limits of their respective lifetimes. Se23+ and Te23+ were unambiguously identified by their characteristic isotopic abundances. Ab initio calculations of the electronic structures of Se23+, Te23+, and LaF3+ show that these molecular trications are metastable with respect to dissociation into fragment ions of Se2++Se+, Te2++Te+, and La2++F+, respectively. Their barrier heights are about 0.49, 0.29, and 0.53 eV, and the equilibrium internuclear distances (bond lengths) are about 0.23, 0.27, and 0.26 nm, respectively. The gas-phase diatomic dications Se22+ and Te22+ were also observed and unambiguously identified. They were found to be long-lived metastable molecules as well, whereas LaF2+ is thermochemically stable. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Quantum chemical calculations on the Li–C2H4 complex have been performed with coupled-cluster and density functional methods. For both methods the electronic ground state of the complex is calculated to be 2B2, with a C2v symmetry equilibrium structure, and the calculated binding energy is quite small (around 2 kcal/mol), and therefore very much basis set dependent. The vibrational spectrum has been calculated at the harmonic approximation, including 13C/12C, 7Li/6Li, and H/D isotopic substitutions. The agreement between experimental and calculated infrared frequencies is correct, except for the low frequency symmetric Li–C stretching mode. These calculations also allow to propose an assignment for the observed C–H/C–D stretching modes. The observed blue-shift of the symmetric CH2 bending mode as well as the red-shift of the antisymmetric CH2 bending, CD2 bending, and C–C stretching modes with respect to the free ethylene have been confirmed by the density functional calculations. The Na...C2H4 complex has been found to be unstable in its 2B2 electronic state. The study of the 2A1 electronic state for both Na...C2H4 and K...C2H4 complexes show that they are at most very weak van der Waals complexes. This result confirms the conclusions of matrix isolation experiments. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vibrational spectra of H2, D2, and HD trapped in solid argon were examined as a function of concentration and sample annealing. Before annealing, increasing the hydrogen concentration only leads to an asymmetrical broadening and a small blue shift of the Q( J) lines assigned to monomeric species. These observations are believed due to formation of low stoichiometry aggregates. After annealing above 35 K, new features—10 cm-1 blue shifted with respect to monomer and small aggregate lines—appear with properties comparable to those reported for solid dihydrogen at 2 K. Force field calculations on aggregates involving two to six molecules (including ortho/para and isotopic mixtures) allow a semiquantitative interpretation of the experimental data, and suggest an aggregation degree larger than six. Finally it is concluded that a possible interpretation of all the data is the formation of microcrystals upon annealing, tied to the low solubility and the high mobility of dihydrogen in solid argon. [ABSTRACT FROM AUTHOR]

The vibration–rotation and pure rotational Raman spectra of H2, HD, and D2 trapped in Ar, Kr, and Xe matrices have been recorded at 9 K. The frequencies which have been measured within an accuracy of 0.3 cm-1 are compared to the results of recent calculations. Except for the S0(0) line of HD the agreement between observed and calculated matrix shifts is satisfactory. The anomalous blue shift observed for the S0(0) line of HD is well interpreted within the rotation translation coupling (RTC) framework. [ABSTRACT FROM AUTHOR]

A thorough topological investigation is used to study the possible isomers of (SO3)(H2XY) complexes, for X=C, Si and Y=O, S. From the topological properties of the isolated monomers, up to five types of isomers can be considered, depending on the nature of the X and Y atoms. After geometry optimisations, three isomers belonging to four different types of geometry were obtained. The natures of the X and Y atoms not only affect the relative stability of the isomers, but also the interaction sites available on the H2XY molecule. For all the systems considered in this study, a covalent structure was obtained from a non-covalent isomeric form involving a double π-hole interaction. Such a covalent structure corresponds to the global minimum for the (SO3)(H2CS), (SO3)(H2SiO) and (SO3)(H2SiS) complexes, and a local minimum for the (SO3)(H2CO) complex. The usefulness of physically-relevant topological tools for a thorough characterisation of the monomers prior to a search for the isomeric forms of a complex is once again demonstrated. [ABSTRACT FROM PUBLISHER]

Spectroscopic properties of 1:n complexes (n = 1, 2, and 3) formed between an oxirane molecule and water clusters have been evaluated using experimental techniques (FTIR spectroscopy using a new supersonic jet experiment coupled to the infrared AILES beamline of synchrotron SOLEIL and also cryogenic neon matrix device) and theoretical approaches (SAPT, ab initio, DFT, and topological analyses). From a systematic comparison between the theoretical results (obtained with both wave function based methods and several newly hydrogen bonded adapted functionals) with the available experimental results on the studied compounds, it was concluded that only the wave function based methods (particularly coupled clusters ones) are able to well describe these compounds, while the newly hydrogen bonded adapted functionals (long-range and/or dispersion-corrected ones and also double hybrids) cannot adequately describe all the spectroscopic properties in a systematic way. The MP2 method, although more expensive than DFT, still offers a reliable method to study both isolated molecules and hydrogen bonded complexes provided the contribution of the dispersion energy in total energy is properly taken into account. The nature of interaction between oxirane and water molecules has been analyzed using the symmetry adapted perturbation theory (SAPT) method. It was evidenced that the water--oxirane interaction corresponds to the hydrogen-bonded systems with a large contribution of the dispersion energy. The nature of the oxirane--water bonding has been studied using two topological methods: atoms in molecules and electron-localization function (ELF). Geometrical structures of the titled complexes were rationalized from the spatial arrangement of ELF attractors. Secondary interaction was also accounted for the bond critical points found at Hoxirane⋯Owater bond paths. [ABSTRACT FROM AUTHOR]

The nature of the bonding between lithium atoms, in low-spin and maximum-spin $$\hbox{Li}_{n} (n {=} 2\hbox{--}6)$$ clusters, was investigated using the topological electron localization function (ELF) approach. The maximum-spin clusters are especially intriguing since their bonding is sustained without having even a single electron pair! Hence this type of bonding had been called “no-pair ferromagnetic-bonding” [Danovich, Wu, Shaik J Am Chem Soc 121:3165 (1999); Glokhovtsev, Schleyer Isr J Chem 33: 455 (1993); de Visser, Danovich, Wu, Shaik J Phys Chem A 106:4961 (2002)]. The following conclusions were reached in the study: (a) In the ground state of Li n , covalent bonding between Li atoms is accounted by the presence of the disynaptic valence basins, which exhibit a significant degree of inter-basin delocalization. (b) Except for the 3Li2 case, the valence basins of all maximum-spin clusters are populated by unpaired electrons. The valence basins are located off Li–Li axis (or Li–Li–Li plane), so that their spatial distribution minimizes the mutual Pauli repulsion and screens the electrostatic repulsion between the Li cores. The inter-basin delocalization is rather high, thereby indicating that the unpaired electrons are virtually delocalized over all the valence basins. (c) The ELF analysis shows that Li atoms in the low-spin clusters are bonded by “two-center two-electron” and “three-center two-electron” bonds. (d) In the maximum-spin species, bonding is sustained by “two-center one-electron” and “three-center one-electron” bonds. The latter picture is complementary to the valence bond picture [Danovich, Wu, Shaik J Am Chem Soc 121 3165 (1999); de Visser, Danovich, Wu, Shaik J Phys Chem A 106: 4961 (2002)], in which the bicentric ferromagnetic-bonding is delocalized over all the short Li–Li contacts, by the mixing of the ionic structures and other nonredundant structures into the repulsive high-spin covalent structure in which all the electrons populate the 2 s atomic orbitals, i.e., the $$2s_a^{1} 2s_b^{1}\, 2s_i^{1}\cdots2s_n^{1}$$ configuration. In such a manner bonding can be sustained from “purely ferromagnetic interactions” without electron pairing. [ABSTRACT FROM AUTHOR]