Van Der Waals Trimers Research Articles

Thermodynamic Properties of van der Waals Dimers and Trimers of Nonpolar Gases and Their Correlation with Lennard-Jones Potential Well Depths.

A method of unifying the equilibrium thermodynamic properties ΔHo and ΔGo relating to the van der Waals dimers and trimers of 15 nonpolar gases (He-Xe, H2, D2, CH4, CF4, ethene, ethane, CO2, SF6, propane, and neopentane) is described. Values of ΔHo and ΔGo, obtained at the reduced temperature Tr = 0.7, show good correlation with the respective Lennard-Jones pair potential well depth, calculated from the monomer critical temperature and the acentric factor. Such relationships present the opportunity to estimate the van der Waals dimer and trimer thermodynamic properties of other nonpolar molecules, and examples of seven such applications are given. It is found that the enthalpies of dimerization and trimerization of the 15 gases are about 21 and 29% of the respective condensation enthalpies, providing information about the thermodynamics of small clusters in relation to liquefaction.

The thermodynamics of the van der Waals dimer and trimer formation in gaseous carbon dioxide and methane and their heterodimers with dinitrogen

The equilibrium thermochemical properties of the dimers and trimers of the greenhouse gases carbon dioxide and methane, as well as their heterodimers with dinitrogen, have been calculated from the CO2 and CH4 equations of state (EOSs) and statistical mechanics. The standard dimer and trimer equilibrium constants, along with the respective ΔHoandΔSo values are reported for CO2 and CH4 between 220 and 260, and 95 and 135 K, respectively, and for the heterodimers between 100 and 250 K. Statistical mechanical calculations based on experimental and computational data of the dimers provide qualitative agreement with the EOS results. The equilibrium thermochemical properties of the CH4 dimer and the CO2–N2 and CH4–N2 heterodimers are calculated using a Morse potential to represent pairwise interactions in an “atom–diatom” model that accounts for both bound and metastable rovibrational states.

Experimental and theoretical investigations of HeNeI2 trimer.

We report on the results of spectroscopic studies of the HeNeI2 van der Waals trimer using the two-step two-color HeNeI2(E0g +, vE = 0-3 ←hν2B0u +, vB = 19 ←hν1X0g +, vX = 0) excitation scheme. The excitation spectra of the HeNeI2(B, E) decay product luminescence and the luminescence spectra of I2 ion-pair states formed after HeNeI2(E) decay have been recorded and analyzed. The HeNeI2(X, B, E) binding energies have been estimated to be less than 111.8 cm-1, 101.6 cm-1, and 117.9 cm-1, respectively. The HeNeI2(B, vB) state decay has been found to be sequential with the formation of the HeI2(B, vB - 1) and NeI2(B, vB - 1) complexes without intermolecular excitation at the first step. An analysis of the HeNeI2(E) decay process based on the relative probabilities of the I2(D0u +) and I2(β1g) formation after decay, as well as vibrational populations of these states obtained from luminescence spectra, has also been performed. Calculations of the HeNeI2(X, B) vibrational energies using potential energy surfaces (PESs) of HeNeI2(X, B) constructed as a sum of the HeNe, HeI2, and NeI2 potentials have been carried out. The values obtained agree well with the experimental estimations, and the observed transitions can be ascribed to the HeNeI2 trimer of a tetrahedral geometry. Besides, a comparison of the PES constructed as a sum of the coupled-cluster single double triple [CCSD(T)] potentials with the PES based on the "direct" CCSD(T) calculations has been performed for the HeNeI2(X) trimer to verify the applicability of this representation to the trimer under study.

Efimov states in asymmetric three-body atomic clusters

The work is devoted to the investigation of the weakly bound three-body atomic clusters. The calculations on the van der Waals trimer 7Li4He2 are carried out using the differential Faddeev equations, which allows us to give accurate binding energies for both the ground and the exited state of the system. The results obtained indicate the Efimov nature of the excited state in this system.

Prediction of a weakly bound excited state of Efimov character in aLi7He24system

We carry out calculations on the van der Waals trimer (LiHe2)-Li-7-He-4 using the mapping method within the frame of hyperspherical coordinates, which allows us to give accurate binding energies and wave functions for both the ground and excited state of the system. When the realistic two-body potentials are adopted, the system presents an excited state which shows Efimov character. We study the range of the interaction strength in which the excited state could exist and find that the state persists within the experiment error band for binding energy of LiHe molecule. We also study the three-body parameter (3BP) of (LiHe2)-Li-7-He-4 system and its relationship with the background scattering length alpha(HeHe). Our calculations demonstrate that the 3BP of (LiHe2)-Li-7-He-4 system is dependent on the value of the scattering length aHeHe, independent of the short-range details of the He-He interaction. The results confirm the prediction of Wang et al. [Phys. Rev. Lett. 109, 243201 (2012)] that the 3BP for a heteronuclear atomic system is universally determined from the van der Waals lengths and the homonuclear scattering length.

Infrared diode laser spectroscopy of the trimers Rg2[sbnd]N2O (Rg = Ne, Ar, and Kr): The ν1 symmetric stretch region of N2O

Rovibrational spectra of the van der Waals trimers Ne2N2O and Ar2N2O are studied in the ν1 region of N2O-monomer using an infrared tunable diode laser spectrometer to probe a pulsed supersonic jet. The band-origins of 20Ne2N2O and Ar2N2O are determined to be ν0=1285.3407(1) and 1285.3312(1)cm−1, respectively. The Q-branch of Kr2N2O is tentatively assigned. The band-origins of RgnN2O (Rg=Ne, Ar, and Kr) are found to shift approximately linearly for zero, one, and two rare gas atoms contained in ν1 vibrational band of N2O-monomer. The incremental shifts observed for Ne2N2O and Ar2N2O are less than those for NeN2O and ArN2O in the ν1 region of N2O-monomer, whereas the opposite is true in the ν3 region of N2O-monomer. The incremental shifts for Rg2N2O (Rg=Ne, Ar, Kr) can be explained by a model based on the Buckingham intermolecular potential.

Evolution of Microwave Spectroscopy at the National Bureau of Standards (NBS) and the National Institute of Standards and Technology (NIST)

This paper describes the beginning and evolution of microwave rotational spectroscopic research starting in 1954 at the National Bureau of Standards (NBS), located at that time in Washington, DC, through the present at NIST in Gaithersburg, MD. David Lide was hired in 1954 to start this research employing Stark modulated waveguide septum cells. When Donald R. Johnson joined the lab in 1968, he developed parallel plate cells coupled with rf and DC discharge methods to study free radicals and transient species. In the mid 1980s Lovas and Suenram constructed a pulsed molecular beam Fourier Transform microwave (FTMW) spectrometer to study hydrogen bonded and van der Waals dimers and trimers. This article describes the types of molecules studied and the type molecular properties derived from these measurements as well as some of the instruments developed for these studies. The two major areas of application described are atmospheric chemistry and molecular radio astronomy.

Hyperspherical study of Ne2Kr and Ne2Xe systems

We have calculated the ground state energies and rotational constants for the Ne2Kr and Ne2Xe systems, as well as their corresponding isotopes using the hyperspherical method. We used B-splines as basis function to expand channel functions and make the knot distribution of B-splines characterize the behavior of the channel function precisely. As a result, the extremely slow convergence of quantum mechanic calculation for these van der Waals trimers containing heavy element is improved greatly. The convergent rotational constants and the isotope shifts are obtained and compared with other theoretical and experimental values. Our results using newly proposed Tang-Toennies (TT) pair-wise potentials are consistent with that of Ernesti and Hutson’s calculation using HFD-B potentials, and give an improved agreement with experimental data.

Infrared spectra of the Ne2–N2O, Ar2–N2O trimers

Spectra of the van der Waals trimers Ar2–N2O and Ne2–N2O are studied in the region of the N2O ν1 fundamental (≈2220cm−1) using a tunable quantum cascade laser to probe a pulsed supersonic expansion from a slit jet nozzle. Improved data are obtained for the dimers Ar–N2O and Ne–N2O, and the Q-branch of Ar3–N2O is tentatively assigned. The vibrational shifts for Nen–N2O are almost exactly linear for n=0–2. However, for Arn–N2O the n=2 band origin is slightly blue-shifted compared to the linear prediction, and the n=3 origin (if correct) is more significantly blue-shifted (by 0.09cm−1).

A theoretical study of Ne3 using hyperspherical coordinates and a slow variable discretization approach

We study theoretically the ground and excited bound states of the bosonic rare gas van der Waals trimer Ne(3). A slow variable discretization approach is adopted to solve the nuclear Schrödinger equation, in which the Schrödinger equation in hyperangular coordinates is solved using basis splines at a series of fixed finite-element methods discrete variable representation hyper-radii. We consider not only zero total nuclear orbital angular momentum, J = 0, states but also J > 0 states. By using the best empirical neon dimer interaction potentials, all the bound state energy levels of Ne(3) will be calculated for total angular momenta up to J = 6, as well as their average root-mean-square radii. We also analyze the wave functions in hyperspherical coordinates for several selected bound states.

Spectroscopic study of the Ne–Xe–NH3 van der Waals trimer

Rotational spectra of the Ne-Xe-NH3 van der Waals trimer were recorded using a pulsed-nozzle, Fourier transform microwave spectrometer. Both a- and b-type transitions of eight isotopologues, namely 20Ne-132Xe-14NH3, 20Ne-129Xe-14NH3, 20Ne-132Xe-15NH3, 20Ne-129Xe-15NH3, 20Ne-131Xe-15NH3, 22Ne-132Xe-15NH3, 22Ne-129Xe-15NH3, and 22Ne-131Xe-15NH3 were measured and assigned. Nuclear quadrupole hyperfine structures arising from the 14N (nuclear spin quantum number I = 1) and 131Xe (I = 3/2) nuclei were detected and analyzed. The determined rotational constants were used to fit structural parameters. A harmonic force field analysis was performed based on centrifugal distortion constants to extract information about vibrational motions of the complex. A comparison of van der Waals bond lengths and stretching force constants between the Ne-Xe-NH3 trimer and the corresponding dimers indicates that non-additive three-body effects are present in the trimer system. Analyses of the 14N and 131Xe nuclear quadrupole coupling constants suggest that the NH3 unit undergoes nearly free internal rotation within the complex and that the presence of Ne has little effect on the orientation of NH3 with respect to the Xe atom.

Tunable diode laser spectroscopy of helium clusters

Helium clusters, HeN-X, containing a probe molecule, X, are studied by infrared spectroscopy for the size range N≈1∼100. Spectra are observed using a supersonic jet expansion and a tunable diode laser source operating in a rapid-scan (sweep integration) mode. The pulsed jet uses a dilute gas mixture of the probe molecule in helium, with relatively high backing pressures (5–50 bar), and a cooled (80–295 K) nozzle. Sensitivity is enhanced by multi-passing the laser beam through the jet with a toroidal mirror system. The clusters are larger than van der Waals dimers and trimers, but smaller than those encountered in the field of helium nanodroplets (N≈103–105). Furthermore, individual cluster sizes are resolved here, but not with nanodroplets, and infrared absorption is detected directly (change in transmitted laser intensity), rather than indirectly (change in cluster fragmentation). Trends in the spectra are described for five probe molecules, X=CO, SiH4, OCS, N2O, and CO2. Superfluid effects dominate for clusters larger than N≈8. Notable results include the unexpected observation of broad oscillations in the effective rotational constants as a function of cluster size.

Hyperspherical coupled channel calculations for the spectra and structure parameters of rare gas trimers NeAr2 and Ne2Ar

We calculate the L=0 vibration energies and rotational constants for the van der Waals trimers 20NeAr2, 20Ne2Ar, and their corresponding isotopologues within the framework of hyperspherical coordinates. The Schrodinger equation in hyperangular coordinates is solved at a series of fixed hyper-radii using B-splines and the resulting coupled hyper-radial equation is solved using the slow variable discretization method developed by Tolstikhin et al. [J. Phys. B 29, L389 (1996)]. Using the special properties of B-splines, we make the knot distributions more precisely, characterizing the behavior of channel functions. Our method improves the convergence greatly. It turns out that our numerical tool works quite well in study of rare gas trimers. Calculations are performed on two kinds of pair potentials, the HFD-B and Tang-Toennies (TT) potentials, and the resultant rotational constants and their isotope shifts are compared with the experimental results obtained from high-resolution spectroscopy. The TT pair potentials give much better agreement with the experimental values for 20Ne2Ar and 22Ne2Ar trimers, while the HFD-B pair potentials give much better agreement with the experimental values for 20NeAr2 and 22NeAr2 trimers.

Rotational Spectra of the Xe−(H2O)2van der Waals Trimer: Xenon as a Probe of Electronic Structure and Dynamics

Rotational spectra of three isotopomers of the Xe-(H2O)2 van der Waals trimer were recorded using a pulsed-nozzle, Fourier transform microwave spectrometer. Nine [nine, four] a-type and twelve [eleven, seven] b-type transitions were measured for the 132Xe-(H2O)2 [129Xe-(H2O)2, 131Xe-(H2O)2] isotopomer. The determined rotational and centrifugal distortion constants were used to extract information about the structure and vibrational motions of the complex. The nuclear quadrupole hyperfine structures due to the 131Xe (nuclear spin quantum number I=3/2) nucleus were also detected. The large value of the off-diagonal nuclear quadrupole coupling constant chiab in particular provides detailed insight into the electronic environment of the xenon atom and the orientations of the water molecules within the complex. An effective structure that best reproduces the experimental 131Xe nuclear quadrupole coupling constants is rationalized by ab initio calculations. An overall goal of this line of work is to determine how the successive solvation of a xenon atom with water molecules affects the xenon electron distribution and its intermolecular interactions. The results may provide molecular level interpretations of 129Xe NMR data from, for example, imaging experiments.

Calculating energy levels of isomerizing tetra-atomic molecules. I. The rovibrational bound states of Ar2HF

A general, six-dimensional computational method for the accurate calculation of rotationally and vibrationally excited states of tetra-atomic molecules is developed. The resulting program is particularly appropriate for molecules executing wide-amplitude motions and isomerizations. An application to the Ar2HF van der Waals trimer is presented in which the HF intramolecular stretching coordinate is separated out adiabatically and is not treated explicitly. Vibrational term values up to about 100 cm−1 with absolute convergence to better than 0.1 cm−1 are reported. These calculations employ more extensive vibrational basis sets and hence consider a much higher density of states than hitherto. States that sample Ar–Ar–HF linear configurations and approach Ar–HF–Ar linear configurations are characterized for the first time. Results for total angular momentum J=0 and 1 provide the first accurate calculations of rotational constants for this system. The rotational constants for the HF bending states of Ar2HF in the ground and first vibrationally excited states of the HF monomer are in good agreement with experiment, confirming the accuracy of the potential used in this work.

Microwave spectra of, and ab initio calculations for, the Ne 2–NH 3 van der Waals trimer

The ground state rotational spectrum of the Ne 2–NH 3 van der Waals trimer was measured in the region between 5 and 19 GHz using a Balle-Flygare type Fourier transform microwave spectrometer. The isotopomers studied include those of 20 Ne 2 paired with NH 3, 15 NH 3 , ND 3, ND 2H, and NDH 2 as well as those containing 20Ne 22Ne and 22 Ne 2 bound to NH 3 and 15 NH 3 . Nuclear quadrupole hyperfine splitting for the 14N containing isotopomers was resolved and included in the fit of the spectroscopic constants. Additional splitting due to the inversion of ammonia was observed for each of the deuterium containing species. This tunnelling splitting cannot be observed for the NH 3 and 15 NH 3 containing isotopomers for nuclear spin statistical reasons. Three ab initio potential energy surfaces were constructed for Ne 2–NH 3 at the CCSD(T) level of theory. Each surface corresponds to a different umbrella angle of NH 3 to simulate the inversion motion. The minimum energy orientation of NH 3 for each of the three potential energy surfaces corresponds to a geometry in which the C 3-axis of NH 3 is aligned perpendicular to the Ne–Ne axis with two of the hydrogen atoms pointed toward the Ne atoms. The potential well depth associated with this orientation is 131 cm −1 (597.2 μH) for the experimental equilibrium geometry of NH 3 with a Ne–Ne bond length of 3.29 Å (fixed) and a Ne–NH 3 bond length of 3.51 Å.

Ab Initio Study on the (OCS)2·CO2van der Waals Trimers

Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] were carried out to study the (OCS)2·CO2 van der Waals trimers. Three barrel-like structures [C1 (two) and C2 symmetry] and three planar (Cs) structures were located on the potential energy surface. Their CBS-MP2/cc-pVXZ (X = D, T, Q) stabilization energies are 1760 (C1), 1514 (C2), 1660 (C1), 1325 (Cs), 1556 (Cs), and 1398 (Cs) cm-1, respectively. The most stable structure (one of the C1 barrel-like isomers) has bond lengths, angles, rotational constants, and dipole moment that agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the rest of the isomers strongly suggests that the experimentally unobserved structures might also be present in the supersonic adiabatic expansion of the gas in the microwave spectroscopic studies as in the case of the (CO2)3 trimer where bot...

Ab initio and DFT studies on van der Waals trimers: the OCS.(CO2)2 complexes.

Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] and density functional theory [B3LYP/6-31G(d,p)] calculations were carried out to study the OCS.(CO2)2 van der Waals trimer. The DFT has proved inappropriate to the study of this type of systems where the dispersion forces are expected to play a relevant role. Three minima isomers (two noncyclic and one cyclic) were located and characterized. The most stable isomer exhibits a noncyclic barrel-like structure whose bond lengths, angles, rotational constants, and dipole moment agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the three isomers, with stabilization energies of 1442, 1371, and 1307 cm-1, respectively, at the CBS-MP2/cc-pVXZ (X = D, T, Q) level, strongly suggests that the two unobserved structures should also be detected as in the case of the (CO2)3 trimer where both noncyclic and cyclic isomers have been reported to exist. The many-body symmetry-adapted perturbation theory is employed to analyze the nature of the interactions leading to the formation of the different structures. The three-body contributions are small and stabilizing for the two most stable structures and almost negligible for the cyclic isomer.

Microwave rotational spectroscopic investigation of the Ar2–NH3van der Waals trimer

Rotational spectra of five isotopomers of the Ar2–NH3 van der Waals trimer were recorded between 3.9 and 21 GHz using a pulsed jet, Balle–Flygare type Fourier transform microwave spectrometer. The observed microwave transitions are b-type transitions within the ground internal rotor state of the complex. 14N nuclear quadrupole hyperfine structures of the rotational transitions were resolved and analyzed. The resulting nuclear quadrupole coupling constants are sensitive to the orientation and large amplitude internal rotation of the NH3 moiety within the trimer. For each of the three deuterated isotopomers, a tunnelling splitting due to the inversion of ammonia was observed. The spectroscopic constants obtained from the spectral fits were used to estimate the structure of the complex and to discuss the internal rotation and inversion dynamics of NH3 within the Ar2–NH3 trimer. The results are compared with previous microwave studies of the Ar–NH3 dimer and Ar3–NH3 tetramer and with a recent combined microwave and ab initio investigation of the Ne2–NH3 trimer.

The ArNe-N 2 O van der Waals trimer: a high resolution spectroscopic study of its rotational spectrum, structure and dynamics

The ArNe-N 2 O van der Waals trimer: a high resolution spectroscopic study of its rotational spectrum, structure and dynamics