Minimum Potential Surface Research Articles

Ab initio vibrational spectrum of [formula omitted

The vibrational spectrum and structure of ( 2 Σ + ) He – MgH 2 + has been investigated using relativistically-corrected UCCSD(T). Vibrational wave functions for low-lying l = 0 states were obtained numerically using a w co-ordinate Eckart–Watson vibrational Hamiltonian, in conjunction with an embedded analytical UCCSD(T) potential. For the ground vibrational state, vibration-averaged Mg–H and Mg–He bond lengths differ by −33 and 53 m a 0 from those corresponding to the ab initio potential surface minimum. Vibration transition moment integrals and associated radiative properties of ( 2 Σ + ) He – MgH 2 + have been calculated using an adapted quadrature scheme, in conjunction with an embedded analytical UCCSD(T) dipole moment function.

Ultrathin liquid films under alternating intermolecular potential fields and capillary force

A unique experimental system is devised that reveals the interplay among capillary force and alternating intermolecular forces. As a consequence of the interplay, untrathin (<10 nm) liquid films, which invariably experience dewetting, can be made stable, leading to a smooth and dropless minimum-potential surface. Theory and experiment show that the film thickness is <1 nm when the film recedes in spite of the capillarity and it is <3 nm when it rises into a cavity.

The gas-phase reaction between hydroxide ion and methyl formate: a theoretical analysis of the energy surface and product distribution.

The potential energy surface for the prototype solvent-free ester hydrolysis reaction: OH- +HCOOCH3 --> products has been characterized by high level ab initio calculations of MP4/6-311 + G(2df,2p)//MP2/6-31 + G(d) quality. These calculations reveal that the approach of an OH- ion leads to the formation of two distinct ion-molecule complexes: 1) the MS1 species with the hydroxide ion hydrogen bonded to the methyl group of the ester, and 2) the MS4 moiety resulting from proton abstraction of the formyl hydrogen by the hydroxide ion and formation of a three-body complex of water, methoxide ion and carbon monoxide. The first complex reacts to generate formate anion and methanol products through the well known B(AC)2 and S(N)2 mechanisms. RRKM calculations predict that these pathways will occur with a relative contribution of 85% and 15% at 298.15 K, in excellent agreement with experimentally measured values of 87% and 13%, respectively. The second complex reacts by loss of carbon monoxide to yield the water-methoxide complex through a single minimum potential surface and is the preferred pathway in the gas-phase. This water-methoxide adduct can further dissociate if the reactants have excess energy. These results provide clear evidence that the preferred pathways for ester hydrolysis in solution are dictated by solvation of the hydroxide ion.

Detection of multiple trap sites in α-deuterated 2-indanone using optically detected magnetic resonance

2-indanone was deuterated by refluxing with D 2O and by acid catalyzed enolization, yielding seven distinct molecular species. Samples containing differing total percent deuteration, as determined from mass spectral data, were extracted during the reaction. Using a simulation program, the hydrogen–deuterium exchange was deemed to be a statistically random process. The optically detected magnetic resonance spectrum showed multiple traps in these partially deuterated samples. Seven well-resolved peaks were observed in one of the transitions, while in the other, deconvolution of the spectra assuming Gaussian line shapes yielded all seven lines. In 2-indanone, the third transition is not observed. Correlations were made between the seven peaks in the two transitions by electron–electron spin double resonance, and the zero-field splittings of each of the multiple traps were determined. The seven molecular species generated in the deuteration were correlated to specific zero-field transitions by analyzing the ODMR of samples which had a higher concentration of specific partially deuterated molecules. An analysis of these energies indicated that the shift was the result of a change in the D value which was about 8.1 MHz per additional deuterium. The E value changed little, −2.6 MHz per added deuterium. In addition, the analysis of the integrated intensities indicated that one of the two out-of-plane geometries of the carbonyl group in 2-indanone was favored over the other. Difference in the zero-point energies of the double minimum potential surface was postulated to be the cause.

Far-infrared spectra and two-dimensional potential energy surface for the ring-bending and ring-twisting vibrations of 5,6-dihydro-4H-thiopyran

5,6-Dihydro-4H-thiopyran has been synthesized and its far-infrared spectrum has been recorded. Eleven ring-bending bands originating at 120.7 cm−1 and four ring-twisting bands originating at 274.5 cm−1 were observed. Twelve sum and difference bands in the 383–397 and 148–166 cm−1 regions were also observed and these facilitated the construction of a detailed energy map including numerous excited vibrational states of the two coupled vibrations. The two-dimensional potential energy surface, which satisfactorily fits the observed data, was determined to be V=9.48 ×104x4−4.13×104x2+1.37×104τ4−1.82×104τ2+1.10 ×105x2τ2, where x and τ are the bending and twisting coordinates, respectively. The minima on the potential energy surface correspond to twisting angles of ±48° (half-chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 1500 cm−1 above the twisted conformation on the potential energy surfaces, and the barrier to planarity was estimated to be 6000 cm−1. Both of these values have large uncertainties since the vibrational data only extend to 800 cm−1 above the potential surface minimum. The relatively low bending energy and high barrier to planarity can both be explained by the low force constant for the C–S–C angle bending.

Electronic and geometric structures of the high- and low-spin multiplets of the Cr(C 6H 6) π complex

We report results of a theoretical investigation of four multiplet spectroscopic states for the open faced Cr(C 6H 6) π complex. The calculations are performed using a spin-polarized density functional method. The optimized structures are obtained in an all-electron description within basis sets of bis zeta+polarization quality. The ground state is found to be the state which has the highest spin multiplicity, namely septet 7A 1. The other, higher-lying, states are 5E 2, 3B 1 and 1A 1. The minimum energies of their potential surfaces are found to be placed respectively at 0.04, 0.30 and 1.02 eV above the potential surface minimum of the ground state. For the four states 7A 1, 5E 2, 3B 1 and 1A 1, the equilibrium distances between Cr and the benzene ring are found to be respectively 2.64, 1.92, 1.62 and 1.55 Å.

Repetitive vibronic structure patterns and the energy gap in the spectra of metal complexes caused by excited state distortions

Two unusual features in the electronic spectra of transition metal compounds which result from excited state distortions are the subject of this paper. The first feature, repetitive patterns of vibronic structure, are interpreted in terms of beats in the time domain between the time-dependent overlaps of two or more modes. The patterns which appear in the emission and absorption spectra of bis(maleonitriledithiolato)palladium are analyzed. The second feature, large gaps separating the emission and absorption spectra of some metal complexes, is a consequence of displacements in two or more normal modes, one of which is not totally symmetric. The active asymmetric b 1g mode of PtCl 4 2− is modeled by a double minimum potential surface. The electronic spectra and the energy gap are calculated by using the split operator technique for numerical integration of the time-dependent Schrödinger equation.

An extended study of the lowest Π bending vibration–rotation spectrum of Ar–HCl by intracavity far infrared laser/microwave double resonance spectroscopy

The lowest Π bending state in Ar–HCl has been more completely characterized through the use of far infrared laser/microwave double resonance spectroscopy. This extended analysis includes a partial reassignment of the far infrared spectra of Ar–H35Cl previously reported by Ray et al. and by Marshall et al., as well as an analysis of the ArH37 Cl spectra. Improved molecular constants have been determined. The value of the rotational constant is now in good agreement with that calculated by Hutson from the M5 double minimum potential surface. Along with the recently reported spectrum of the Σ bending vibration, this extended analysis provides strong evidence for the existence of two minima character in the intermolecular potential surface of Ar–HCl.

Luminescence behavior of a metallized semiconductor powder

A version of the dead-layer model, originally developed for single crystals, is derived to describe the luminescence behavior of a metallized Zn(Cd)S:Ag phosphor powder. Electron-hole separation occurs only with excitation events capable of leading to free photoelectrons. The potential for the electrons in the deposited ‘‘metal’’ layer may deviate from the metal work function, when chemical modification, e.g., oxidation, of the metal occurs subsequent to deposition. We call particular attention to the dramatic difference in photoluminescence lifetime in the Zn(Cd)S under laser exposure conditions compared with lower intensity xenon flash exposure. Both this observation and the incomplete quenching are rationalized in terms of a double minimum potential surface for hole trapping at the Ag(I) centers.

Search for possible crystalline packings of polymethylene chains within the framework of the atom-atom approach

The method of atom-atom potential functions has been used to calculate the energies and equilibrium structural parameters (at 0°K) of various crystal modifications of PE. It is shown that the triclinic modification gives rise to the global minimum potential surface. The selection of the stable structure under normal conditions is determined by the entropy factor. The calculations have shown that two rhombic modifications can exist in ordinary crystalline specimens of PE; in one of these, the symmetry axis 2 1 is parallel to the length a, and in the other, it is parallel to the short b crystallographic axis. Possible routes for polymorphic transitions in crystalline PE are considered.

Virial expansion of the second layer in physical adsorption. An ab initio calculation for helium on argon crystal

A theory of localized and nonlocalized adsorption is given by applying the virial expansion theorem to the second layer. An ab initio calculation of the thermodynamic functions is carried out for the system of helium on an argon crystal. The gas-solid interaction potential is calculated using lattice summation. The periodic potential along the directions parallel to the solid surface is described by a polynomial φn (x/d)/V0 = 1−[1−(2x/d)2]n within a site spacing d, where V0 is the height of potential barrier and n is an integer to be determined from the minimum potential surface. The values of n used for the first and second layers in the present work are 7 and 4, respectively. The second virial coefficient under the external potential is very well approximated by that of the two dimensional gas in free space. The Boyle temperature is 16.3°K for the system which is close to the experimental temperatures (10–20°K) of Ross and Steele. The deviation from the ideality due to the second virial coefficient is small in the experimental temperature ranges; in the isosteric heat, the contribution coming from the second virial coefficient is less than 10% of the total heat. The theory is in excellent agreement with experiment of Ross and Steele.

Calculated frequencies and intensities associated with coupling of the proton motion with the hydrogen bond stretching vibration in a double minimum potential surface

The influence of the coupling of the proton movement and the H bond stretching vibration in a double minimum potential energy surface on the energy levels, transitions, induced dipole moments and polarisabilities is calcualted ab initio as a function of an electric field for the H5O+2 system. The high polarisability of the hydrogen bonds remains to a large extent unchanged due to the coupling. New types of transitions occur, particularly when the tunnelling frequency and the frequency of the bond stretching vibration are comparable in size. Especially in this case numerous Fermi resonances occur due to the shift of the energy levels in the electric field, which leads to a considerable increase in the number of transitions. It is shown that the change of the frequencies of the transitions due to the induced dipole interaction of the bonds with fields from their environment is a decisive cause of the variety of energy level differences observed as a continuous absorption in the i.r. spectrum of such systems.