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We apply a variant of the Hirshfeld density partitioning technique, HI, to calculate the atomic charges and decompose the dipole moments into the part due to the charges and the induced dipoles developed on each atom for three different transition metal (TM) containing molecules. Additionally, the α and β spin densities are treated separately developing a new variant (spin-adapted HI) of the fractional occupation HI version proposed recently. We also study the dependence of HI charges on the atomic state of the TM employed in the promolecule. The VO case exhibits a strong dependence of the atomic charge on the V or V+ state used. Although the bonding in the ground high spin electronic state and the first excited low spin state in TiO and VO is essentially identical, the dipole moments differ significantly and we find that this is due entirely to the σ electron distribution localized on the transition metal. Finally, the mechanism for the rapid change of the dipole moment of HScO upon bending is confirmed to occur mainly due to the induced atomic charges. [ABSTRACT FROM AUTHOR]

We report the dipole and quadrupole moments of the halogenated acetylenes calculated using large basis sets and the SCF, DFT(B3LYP), and CCSD methods, and we analyze the charge density using the Hirshfeld and Hirshfeld-I techniques. The atomic charges, dipoles, and quadrupoles resulting from the Hirshfeld-I analysis are used to interpret the unusually small molecular dipole moments in the sequence as well as the molecular quadrupole moments. The very small dipoles obtain for two reasons. First, the dipole moment associated with the σ and π electron densities is comparable in magnitude and opposite in direction. Second, the charge and induced dipole contributions for ClCCH, BrCCH, and ICCH have opposite signs further reducing the molecular dipoles. The molecular quadrupole moments are the sum of a charge, atomic dipole, and in situ quadrupole terms, and are dominated by the atomic dipoles and in situ quadrupoles with the charge contributions playing an unexpectedly minor role. [ABSTRACT FROM AUTHOR]

We calculated the dipole moment function for the ground 3Σ-(mS=+1) state of the open shell molecules, NF, NCl, and NBr, and analyzed it in terms of the charge and spin distribution and the induced atomic dipoles using the Hirshfeld partitioning of the electron density. The smallest dipole moment (0.026ea0) obtains with NF, in which the atoms have the largest difference in electronegativity, while the dipole moments in NCl and NBr are 0.441ea0 and 0.506ea0, respectively. All dipoles have the N-X+ polarity. In the σ system α spin electrons flow from N to the halogen while β spin electrons flow in the opposite direction and interestingly from both the σ and the π systems of the halogen to the σ system of N. In NF the number of β spins lost by F is essentially equal to the number of α spins gained and the atomic charges are essentially 0. The small dipole in NF is the result of a slight imbalance in the induced atomic dipoles. For NCl and NBr the halogen loses more β spins than it gains α spins resulting in the polarity N-X+. It is interesting that at equilibrium N gained electrons in the π system while the halogen lost π electrons relative to the separated atoms. This however is not back donation in the usual sense because the electrons gained by N have α spin while those lost by the halogen have β spin. Detailed examination of the spin flow shows that the excess α electrons in the π system of N come from an intra-atomic transfer from the N σ system. The induced atomic dipole moments essentially cancel at all internuclear separations and the polarity of the dipole moment accurately reflects the molecular charge distribution. [ABSTRACT FROM AUTHOR]

The dipole and quadrupole moment functions of the hydrogen halides are calculated using a large polarized basis and correlated wavefunctions and compared to experiment and previous calculations. These functions are analyzed in terms of local moments constructed using the Hirshfeld method. The dipole moment is the sum of the functions qHR+μH and μX with qH being the charge on the hydrogen atom, R the internuclear separation, μH and μX the atomic dipoles on the hydrogen and halogen atoms. We find that qHR+μH is always positive and has a maximum at bond lengths larger than the equilibrium. In HF, μF is slightly positive at the maximum in qHR+μH and has little effect on the resultant maximum in the dipole moment function (DMF). μCl, μBr, and μI, on the other hand, are increasingly more negative at the maximum of qHR+μH and have a profound effect on the width of the maximum of the resulting DMF, successively broadening it and completely eliminating it at HI. The quadrupole moment function (QMF) (with the halogen as origin) is given by Θ(HX)=ΘHXproto+δΘX+δΘH+2μHR+qHR2, where ΘHXproto is the quadrupole moment of the separated atoms (the halogen in this instance) and δΘX+δΘH the change in the in situ quadrupole moments of the halogen and hydrogen atoms. The maximum in the QMF and its slope at equilibrium are determined essentially by 2μHR+qHR2, which is known once the DMF is known. δΘX+δΘH is always negative while ΘHXproto is positive, so one can approximate the molecular quadrupole moment to within 10% as Θ(HX)>ΘHXproto+2μHR+qHR2. [ABSTRACT FROM AUTHOR]

High resolution coherent anti-Stokes Raman spectroscopy has been used to study the symmetric CH stretching mode of methyl radical formed by UV laser photolysis of CH3I cooled in a free jet expansion. Spectra obtained under near-nascent conditions (∼3 to ∼6 collisions) show that most of the CH3 product is formed in the ground vibrational state, with little excitation seen in the ν2 out-of-plane bending coordinate [v2=1/v2=0, population ratio 0.27(10)]. This is in accord with recent theoretical calculations favoring slow, adiabatic CH3 relaxation from a pyramidal to planar structure as the C–I bond breaks. Extensive N,K rotational structure is resolved and the distributions obtained lend support to those deduced for nascent CH3 by Chandler and co-workers from modeling of unresolved resonance enhanced multiphoton ionization spectra. The results are consistent with conservation during dissociation of CH3 ortho-para nuclear spin forms and of K spinning angular momentum about the symmetry axis. CH3 product tumbling motion is slightly greater (1–2 units of angular momentum) than predicted theoretically. Rapid collisional excitation of higher rotational levels is seen and the rich spectrum observed after ∼50 collisions has been analyzed to give improved or new vibrational-rotational parameters for 1000 and 1100 states. The 1100 band origin is determined to be 2996.21(4) cm-1, yielding -8.23(5) cm-1 for the x12 anharmonicity constant. [ABSTRACT FROM AUTHOR]

In this report we use the electron density difference function, δη(r), to present an interesting and useful interpretation of several molecular properties. For example, all of the electric multipole moments of a molecule may be written as M[sub αβ...γ][sup molecule]=M[sub αβ...γ][sup atoms]+M[sub αβ...γ][sup cr] where M[sub αβ...γ][sup atoms] is the sum of the moments of the non-interacting atoms and M[sub αβ...γ][sup cr] depends on δη(r) and is the contribution to M[sub αβ...γ][sup molecule] due to charge redistribution upon molecule formation. This interpretation has implications for attempts to represent the multipole moments of an in situ charge distribution by an array of algebraic point charges. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Employing correlation consistent basis sets of quadruple-zeta quality and applying both multireference configuration interaction and single-reference coupled cluster methodologies, we studied the electronic and geometrical structure of the [V,O,H]0,+ species. The electronic structure of HVO0,+ is explained by considering a hydrogen atom approaching VO0,+, while VOH0,+ molecules are viewed in terms of the interaction of V+,2+ with OH-. The potential energy curves for H-VO0,+ and V0,+-OH have been constructed as functions of the distance between the interacting subunits, and the potential energy curves have also been determined as functions of the H-V-O angle. For the stationary points that we have located, we report energies, geometries, harmonic frequencies, and dipole moments. We find that the most stable bent HVO0,+ structure is lower in energy than any of the linear HVO0,+ structures. Similarly, the most stable state of bent VOH is lower in energy than the linear structures, but linear VOH+ is lower in energy than bent VOH+. The global minimum on the potential energy surface for the neutral species is the X3A″ state of bent HVO, although the X5A″ state of bent VOH is less than 5 kcal/mol higher in energy. The global minimum on the potential surface for the cation is the X4Σ- state of linear VOH+, with bent VOH+ and bent HVO+ both more than 10 kcal/mol higher in energy. For the neutral species, the bent geometries exhibit significantly higher dipole moments than the linear structures. [ABSTRACT FROM AUTHOR]

The linear TiCH molecule is studied by ab initio quantum mechanical calculations using quantitative basis sets and highly correlated computational methods. Potential energy curves along the Ti2CH coordinate have been computed to obtain a better understanding of molecular formation in eight low-lying states of the molecule. Total energies, dissociation energies (with respect to Ti +CH), equilibrium distances, and dipole moments are reported. Simple valence bond Lewis diagrams are used to interpret the nature of the bonding in all of the states studied. [ABSTRACT FROM AUTHOR]

Double resonance spectra have been obtained for nitrogen and benzene molecules in the low density region of a molecular beam by use of stimulated Raman pumping followed by ionization probing. For N2 the photoionization step was via a 2+2 resonantly enhanced multiphoton ionization process using the a1Πg(v=2)←X1Σg(v=1) two-photon resonance. The detection limit is 103–104 times lower than for the direct optical measurement of a stimulated Raman signal. For benzene, a more efficient 1+1 ionization step permitted measurements at state densities less than 1010 cm-3 and the Raman step yielded linewidths of 30 MHz for the Q-branch lines of the weak Raman active ν16 fundamental. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]