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1.
Rotational spectra of the weakly bound He-HCCCN and He-DCCCN van der Waals complexes were observed using a pulsed-nozzle Fourier-transform microwave spectrometer in the 7-26-GHz frequency region. Nuclear quadrupole hyperfine structures due to the 14N and D nuclei (both with nuclear-spin quantum number I = 1) were resolved and assigned. Both strong a and weaker b-type transitions were observed and the assigned transitions were used to fit the parameters of a distortable asymmetric rotor model. The dimers are floppy, near T-shaped complexes. Three intermolecular potential-energy surfaces were calculated using the coupled-cluster method with single and double excitations and noniterative inclusion of triple excitations. Bound-state rotational energy levels supported by these surfaces were determined. The quality of the potential-energy surfaces was assessed by comparing the experimental and calculated transition frequencies and also the corresponding spectroscopic parameters. Simple scaling of the surfaces improved both the transition frequencies and spectroscopic constants. Five other recently reported surfaces [O. Akin-Ojo, R. Bukowski, and K. Szalewicz, J. Chem. Phys. 119, 8379 (2003)], calculated using a variety of methods, and their agreement with spectroscopic properties of He-HCCCN are discussed.  相似文献   

2.
Pure rotational transitions of the weakly bound complex He-N(2)O and three minor isotopomers (He-(14)N(15)NO, He-(15)N(14)NO, and He-(15)N(15)NO) were measured in the frequency region from 6 to 20 GHz. Predictions for the microwave transition frequencies were based on the infrared work by Tang and McKellar [J. Chem. Phys. 117, 2586 (2002)]. In the case of (14)N containing isotopomers, nuclear quadrupole hyperfine structure of the rotational transitions was observed and analyzed. The resulting spectroscopic parameters were used to determine geometrical and dynamical information about the complex. An ab initio potential energy surface was calculated at the coupled cluster level of theory with single and double excitations and perturbative inclusion of triple excitations. This surface was constructed using the augmented correlation consistent polarized valence triple zeta basis set for all atoms with the inclusion of bond functions for the van der Waals bond. Bound state calculations were done to determine the energies of low-lying rovibrational levels that are supported by the potential energy surface. The resulting transition energies agree with the experimental values to 1% or better.  相似文献   

3.
Rotational spectra for the normal and four isotopically substituted species of the 1:1 complex between methyl fluoride (H3CF) and carbonyl sulfide (OCS) have been measured using Fourier-transform microwave spectroscopy in the 5-16 GHz frequency region. The observed spectra fit well to a semirigid Watson Hamiltonian, and an analysis of the rotational constants has allowed a structure to be determined for this complex. The dipole moment vectors of the H3CF and OCS monomers are aligned approximately antiparallel with a C...C separation of 3.75(3) A and with an ab plane of symmetry. The values of the Pcc planar moments were found to be considerably different from the expected rigid values for all isotopologues. An estimate of approximately 14.5(50) cm-1 for the internal rotation barrier of the CH3 group with respect to the framework of the complex has been made using the Pcc values for the H3CF-OCS and D3CF-OCS isotopic species. Two structures, very close in energy and approximately related by a 60 degrees rotation about the C3 axis of the methyl fluoride, were identified by ab initio calculations at the MP2/6-311++G(2d,2p) level and provide reasonable agreement with the experimental rotational constants and dipole moment components.  相似文献   

4.
Pure rotational spectra of five isotopomers of the 1:1 weakly bound complex formed between carbonyl sulfide and trifluoromethane (TFM) have been measured using Fourier transform microwave spectroscopy. The experimental rotational constants and dipole moment components are consistent with a structure of C(s) symmetry in which the dipole moment vectors of OCS and HCF(3) are aligned antiparallel and at an angle of about 40 degrees and with a center of mass separation of 3.965(26) A. The derived H...O distance is 2.90(5) A, which is up to 0.6 A longer than is seen in other similar TFM complexes exhibiting C-H...O interactions. Ab initio calculations at the MP2/6-311++G(2d,2p) level give a structure with rotational constants that are in reasonable agreement with those of the normal isotopic species.  相似文献   

5.
The rotational spectra of the van der Waals complex of carbonyl sulfide loosely bound to two helium atoms were investigated using a cavity Fourier transform microwave spectrometer and a microwave–microwave double resonance spectrometer. The lowest rotational transitions of seven isotopomers, namely those containing the 16O12C32S, 16O12C34S, 16O13C32S, 16O12C33S, 18O13C32S, 16O13C34S, and 17O13C32S subunits, were studied. Hyperfine structures due to the quadrupolar 33S and 17O nuclei were also observed and analyzed. The search for higher J transitions was not successful and possible causes are discussed. Information about the OCS–He2 cluster inferred from the measurements is presented and discussed.  相似文献   

6.
We discuss the effect of the break-up of weakly bound nuclei on the fusion cross section of these projectiles with light, medium and heavy mass targets, at energies close to the Coulomb barrier. The discussion is based mostly on data obtained by our group in collaborative experiments. We also show that the break-up process at near and sub-barrier energies is responsible for the vanishing of the usual threshold anomaly of the optical potential.  相似文献   

7.
The rotationally resolved S(1) <-- S(0) electronic spectrum of the water complex of p-difluorobenzene (pDFB) has been observed in the collision-free environment of a molecular beam. Analyses of these data show that water forms a planar sigma-bonded complex with pDFB via two points of attachment, a stronger F---H-O hydrogen bond and weaker H---O-H hydrogen bond, involving an ortho hydrogen atom of the ring. Despite the apparent rigidity of this structure, the water molecule also is observed to move within the complex, leading to a splitting of the spectrum into two tunneling subbands. Analyses of these data show that this motion is a combined inversion-internal rotation of the attached water, analogous to the "acceptor-switching" motion in the water dimer. The barriers to this motion are significantly different in the two electronic states owing to changes in the relative strengths of the two hydrogen bonds that hold the complex together.  相似文献   

8.
Vibration-rotation spectra of the OCS-(CO(2))(2) van der Waals complex were studied by means of direct infrared absorption spectroscopy. Complexes were generated in a supersonic slit-jet apparatus, and the expansion gas was probed using a rapid-scan tunable diode laser. Infrared bands were observed for two different isomeric forms of the complex. A relatively strong band centered at 2058.799 cm(-1) was assigned to the most stable isomer, which has a barrel-shaped geometry and is already known from microwave spectroscopy. A weaker infrared band centered at 2050.702 cm(-1) was assigned to a new isomeric form, observed here for the first time, which was expected on the basis of ab initio calculations. Infrared bands for seven isotopomers were recorded in an attempt to quantify the structure of the new isomer. Because it has no symmetry elements, nine parameters are needed to fully define the geometry. It was possible to determine six of these which define the relative position of the OCS monomer with respect to the CO(2) dimer fragment in the complex while the remaining three were fixed at their ab initio values. Similarities and differences between the faces of the two isomers of OCS-(CO(2))(2) and the associated dimers are discussed.  相似文献   

9.
The Fourier transform microwave spectra of the various isotopologs of the weakly bound complex of carbon dioxide with the most abundant molecule in the atmosphere, nitrogen, have been measured. The structure of the complex has been determined and evidence for the inversion of the N(2) is presented. The molecule is T-shaped, with the OCO forming the cross of the T, a structure consistent with that deduced from a previous rotationally resolved infrared experiment. A significant wide-amplitude bending motion of the N(2) is deduced from the values of the (nearly identical) nuclear quadrupole coupling constants of the nitrogen nuclei. The spectroscopic results are compared with high-quality ab initio calculations. We examine the consequences of the N(2) CO(2) complex formation in the atmosphere upon the greenhouse warming potential of carbon dioxide.  相似文献   

10.
The photoelectron spectra of ozone have been recorded and the first three electronic band systems reassigned on the basis of observed vibrational structure and calculations reported in the literature. The systems X?,Ã, and B? at 12.75, 13.03, and 13.57 eV are assigned as 2A1, 2A2, and 2B2, respectively.  相似文献   

11.
The spherical limit of a single particle in the field of three nuclei is shown to exhibit a variety of phenomena associated with the stability of the various extrema involved. Some consequences with respect to the general problem of detecting broken symmetry solutions of variational problems are pointed out.  相似文献   

12.
We have studied the effect of humidity on the electronic properties of DNA base pairs. We found that the hydrogen links of the nucleobases with water molecules lead to a shift of the pi electron density from carbon atoms to nitrogen atoms and can change the symmetry of the wave function for some nucleobases. As a result, the orbital energies are shifted which leads to a decrease in the potential barrier for the hole transfer between the G-C and A-T pairs from 0.7 eV for the dehydrated case to 0.123 eV for the hydrated. More importantly, the pi electron density redistribution activated by hydration is enhanced by the intrastrand interactions. This leads to a modification of the nucleobase chemical structures from the covalent type to a resonance structure with separated charges, where some pi electrons are not locked up into the covalent bonds. Within the (G-C)(2) sequences, there is overlapping of the electronic clouds of such unlocked electrons belonging to the stacked guanines, that significantly increases the electron coupling between them to V(DA)=0.095 eV against the V(DA)=0.025 eV for the dehydrated case. Consequently, the charge transfer between two guanines within the (G-C)(2) sequences is increased by 250 times due to hydration. The presence of nonbonded electrons suppress the band gap up to approximately 3.0 eV, that allows us to consider DNA as a narrow band gap semiconductor.  相似文献   

13.
New ab initio potential energy surfaces for the (2)Pi ground electronic state of the Ar-SH complex are presented, calculated at the RCCSD(T)/aug-cc-pV5Z level. Weakly bound rotation-vibration levels are calculated using coupled-channel methods that properly account for the coupling between the two electronic states. The resulting wave functions are analyzed and a new adiabatic approximation including spin-orbit coupling is proposed. The ground-state wave functions are combined with those obtained for the excited (2)Sigma(+) state [D. M. Hirst, R. J. Doyle, and S. R. Mackenzie, Phys. Chem. Chem. Phys. 6, 5463 (2004)] to produce transition dipole moments. Modeling the transition intensities as a combination of these dipole moments and calculated lifetime values [A. B. McCoy, J. Chem. Phys. 109, 170 (1998)] leads to a good representation of the experimental fluorescence excitation spectrum [M.-C. Yang, A. P. Salzberg, B.-C. Chang, C. C. Carter, and T. A. Miller, J. Chem. Phys. 98, 4301 (1993)].  相似文献   

14.
We have previously reported that, in electron capture dissociation (ECD), rupture of strong intramolecular bonds in weakly bound supramolecular aggregates can proceed without dissociation of weak intermolecular bonds. This is now illustrated on a series of non-specific peptide-peptide dimers as well as specific complexes of modified glycopeptide antibiotics with their target peptide. The weak nature of bonding is substantiated by blackbody infrared dissociation, low-energy collisional excitation and force-field simulations. The results are consistent with a non-ergodic ECD cleavage mechanism.  相似文献   

15.
The anharmonicity of weakly bound complexes is studied using the vibrational self-consistent field (VSCF) approach for a series of metal cation dihydrogen (M(+)-H(2)) complexes. The H-H stretching frequency shifts of M(+)-H(2) (M(+) = Li(+), Na(+), B(+), and Al(+)) complexes are calculated with the coupled-cluster method including all single and double excitations with perturbative triples (CCSD(T)) level of theory with the cc-pVTZ basis set. The calculated H-H stretching frequency of Li(+)-H(2), B(+)-H(2), Na(+)-H(2), and Al(+)-H(2) is red-shifted by 121, 202, 74, and 62 cm(-1), respectively, relative to that of unbound H(2). The calculated red shifts and their trends are in good agreement with the available experimental and previously calculated data. Insight into the observed trends is provided by symmetry adapted perturbation theory (SAPT).  相似文献   

16.
A considerable fraction of weakly bound complexes is shown to occupy states metastable with respect to spontaneous decay, even at normal temperature in an equilibrium gas of moderate density. Collisionless decay of molecules results in anomalous broadening of spectral lines of hydrogen-bonded or van der Waals complexes.  相似文献   

17.
The microwave spectrum of tellurophene, C4H4Te, has been recorded between 8 and 27 GHz. The rotational constants obtained from least-squares fitting of low-J lines are:
Figures in parentheses are uncertainties in last place of decimal (95% confidence limit). Corresponding inertial defects are 0.094 (7), 0.095 (6) and 0.097 (10) amu A2 respectively, indicating the essential planarity of the molecule.Stark shift measurements correspond to a dipole moment of μa = 0.619 x 10?30 C m (0.186 D).An approximate structure, based on bond lengths of CC and CH bonds adopted from related molecules, implies that r (Te - C) = 205.5 pm and ? C (5) TeC (2) = 82.53°.  相似文献   

18.
The microwave spectra of the four major isotopic species of the unstable molecule selenoformaldehyde have been assigned. The limited number of isotopic variants yields the following structure: r(CSe) = 175.9 pm, ∠(HCH) = 120.4° and r(CH) assumed to be 109 pm. The dipole moment is 1.41 ± 0.01 D.  相似文献   

19.
J. Homer  R.R. Yadava 《Tetrahedron》1973,29(23):3853-3855
Vinyl bromide-benzene/p-xylene and acrylonitrile-benzene/p-xylene complexes have been studied by measuring the 1H chemical shifts induced in the vinyl protons by the aromatic molecules. Evidence is presented that indicates that the complexes are 1:1, and values for the equilibrium quotient, Kx, and the full induced shift Δc, are deduced for each proton; similar values of Kx are obtained for each of the three viny] protons. It is shown that the molecular interactions are specific. However, the complexes cannot be considered to have fixed geometry, but molecules involved take up a variety of relative orientations.  相似文献   

20.
In this investigation, reaction channels of weakly bound complexes CO2…HF, CO2…HF…NH3, CO2…HF…H2O and CO2…HF…CH3OH systems were established at the B3LYP/6‐311++G(3df,2pd) level, using the Gaussian 98 program. The conformers of syn‐fluoroformic acid or syn‐fluoroformic acid plus a third molecule (NH3, H2O, or CH3OH) were found to be more stable than the conformers of the related anti‐fluoroformic acid or anti‐fluoroformic acid plus a third molecule (NH3, H2O, or CH3OH). However, the weakly bound complexes were found to be more stable than either the related syn‐ and anti‐type fluoroformic acid or the acid plus third molecule (NH3, H2O, or CH3OH) conformers. They decomposed into CO2 + HF, CO2 + NH4F, CO2 + H3OF or CO2 + (CH3)OH2F combined molecular systems. The weakly bound complexes have four reaction channels, each of which includes weakly bound complexes and related systems. Moreover, each reaction channel includes two transition state structures. The transition state between the weakly bound complex and anti‐fluoroformic acid type structure (T13) is significantly larger than that of internal rotation (T23) between the syn‐ and anti‐FCO2H (or FCO2H…NH3, FCO2H…H2O, or FCO2H…CH3OH) structures. However, adding the third molecule NH3, H2O, or CH3OH can significantly reduce the activation energy of T13. The catalytic strengths of the third molecules are predicted to follow the order H2O < NH3 < CH3OH. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006  相似文献   

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