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Microwave and infrared spectroscopies help study the internal dynamics and interactions between monomers of weakly bound complexes. High resolution microscopic studies of rare gas complexes of HCCH reveal that they have an energy configuration with a T-shaped minimum, are nonrigid and yield complex rotation-vibration spectra. Tunneling motion in homogeneous dimers, which enables the interchange of bonding roles between the monomers, is studied using spectroscopic techniques. In heterogeneous dimers, one of the subunits exhibits its own inversion motion or internal rotation. Ab initio electronic structure calculation supports the spectroscopic results on geometry and tunneling dynamics of dimers.

We present a phenomenological model to account for the observations of the hydrogen interchange tunneling at N=v1+v2=1–3 of the free (v1) and the bound (v2) HF stretches of (HF)2. Good agreement is obtained between calculations and observations at the levels (v1,v2) of v1-v2=±1 for both N=1 and 3, suggesting that the observed hydrogen tunneling splittings in these levels are direct rather than the results of many extraneous perturbations. The model also predicts well the ratios of the rates of vibrational predissociation at these states, in addition to the splittings. We attribute the unusually large vibrational dependence of the splittings upon valence bond excitation to the result of interbond coupling between the two HF local oscillators. Significant differences in the vibrational energy of the level (1,2) as well as the tunneling splitting at (2,0) between observation and prediction are, however, noted. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The equilibrium structure for the ground state of the HNCN radical is calculated at the levels of the self-consistent field theory (SCF), the second-order Mo\ller–Plesset perturbation approximation (MP2), and the full single and double excitation coupled cluster theory including all connected triples in a noniterative manner [CCSD(T)], using various extended basis sets starting from 6–311 G(d,p). At the CCSD(T) level, the outer C–N bond is more than 0.1 Å shorter than the inner one and the N–C–N group departs from linearity by 6°. The total N–C–N length is in good agreement with the experimental value [Herzberg and Warsop, Can. J. Phys. 41, 286 (1963)], however, the H–N–C angle is about 6° smaller. The N–H bond is very close to a normal N–H bond but is about 0.2 Å smaller than the experimental estimate. Except for the smaller H–N–C angle, the geometrical parameters for HNCN closely parallel those for the triplet HCCN molecule. The dipole moment, harmonic frequencies, electric quadrupole, and Fermi contact coupling constants of HNCN are also calculated. The calculated harmonic frequencies confirm the preliminary assignments of Wu, Hall, and Sears [J. Chem. Soc. Faraday Trans. 89, 615 (1993)]. The quadrupole coupling constants for the inner and outer N atoms are comparable, implying a complex pattern of hyperfine split components in the lowest rotational transitions. The present calculation may thus serve as a useful guide for the interpretation of the rotational spectrum. [ABSTRACT FROM AUTHOR]

The equilibrium structure of the NH3 dimer is investigated using large, efficient basis sets, 6–311+G(3d,2p) and [7s5p3d,4s1p] extended with bond functions, at the second-order Mo\ller–Plesset perturbation approximation (MP2) and higher levels. Intermolecular energies and optimized dimer structures are obtained with the full counterpoise correction for the basis set superposition error. The stabilities of two possible equilibrium structures, one containing a nearly linear hydrogen bond with Cs symmetry and the other a cyclic configuration with C2h symmetry, are examined. In a basis without bond functions, the Cs structure is found more stable. As bond functions are added, however, the C2h structure becomes more stable. This establishes the importance of the dispersion energy which is disproportionally underestimated for the C2h structure in a purely nucleus-centered basis. The stability of the C2h structure relative to the Cs is retained at the higher levels up to the complete forth order (MP4SDTQ). The minimum energy path connecting the two equivalent Cs structures via the C2h structure is calculated. The resulting potential curves are extraordinarily flat in a broad region around the C2h structure but rise steeply upon approaching the Cs structure containing a nearly linear hydrogen bond, indicating that the donor–acceptor interchange barrier is absent in the NH3 dimer. The equilibrium structure for the NH3 dimer found in the present study probably has the cyclic form with C2h symmetry. [ABSTRACT FROM AUTHOR]

Optical spectra recorded with Ar and I2 in a He expansion exhibit fluorescence from an excitation continuum through a broad region of the discrete B←X transitions of I2 and I2–Ar. This fluorescence emanates from B-state I2 and arises from excitations of a bimolecular I2–Ar van der Waals complex. These results were obtained in order to test a proposed mechanism for the one-atom cage effect in I2–Ar, whereby continuum excitation to the repulsive Πu state precedes coupling onto the B state, dissociation of the complex, and fluorescence from B-state I2. The variation of the relative intensity of the observed fluorescence with excitation wavelength can be adequately reproduced with this model, but the Πu←X transition is much too weak to explain the observed absolute intensities. We consider the possible existence of a linear I2–Ar isomer in the expansion along with the well-documented T-shaped isomer. Large geometry changes for the linear isomer upon B←X excitation would result in highly dispersed Franck–Condon factors and thus split this stronger transition over a continuum. Both absolute intensities and wavelength dependences observed for fluorescence from continuum excitation fit well to the linear isomer model. Linear isomers could also be responsible for the one-atom cage effect observed at higher excitation energies. [ABSTRACT FROM AUTHOR]

New experimental results on the structural and dynamical properties of NH3 dimer are reported in this work. J=1–0, K=0 transitions of 14NH3–15NH3, 15NH3–14NH3, ND3 dimer, and ND3–ND2H have been measured at high resolution and 14N electric quadrupole coupling constants are reported for each of these species. The NH3 subunits comprising the dimer are inequivalent. The quadrupole coupling constant associated with the first ammonia subunit eqQ1aa, is measured in 14NH3–15NH3 [-627(8)kHz], in ND3 dimer [-531(15) kHz], and in ND3–ND2H [-991(18) kHz]. For the other subunit, eqQ2aa is reported in 15NH3–14NH3 [892(8)kHz], in ND3 dimer [745(13) kHz], and in NH3–ND2H [1013(18) kHz]. These numbers can be used to estimate the vibrationally averaged polar angles of these isotopomers of NH3 dimer. The result is (including the primary isotopomer) θ1 for 14NH3–14NH3 is 48.6°, for 14NH3–15NH3 is 48.7°, for ND3 dimer is 49.6° and for ND3–ND2H is 45.3°; while θ2 for 14NH3–14NH3 is 64.5°, for 15NH3–14NH3 is 64.3°, for ND3 dimer is 62.6°, and for ND3–ND2H is 65.8°. The remarkable invariance of these values rules out the possibility of large vibrational averaging or tunneling averaging in this system and establishes that the angles θ1=49° and θ2=65° are near equilibrium. The isotope effect in thecomponent of the electric dipole moment along the a inertial axis μa, is shown to correlate well with the trend in polar angles given by the quadrupole coupling constants. The absence of interchange tunneling effects in the observed states of NH3 dimer implies that these states are asymmetrically excited internal rotor states of the complex. These experimental structural results are in disagreement with all previous theoretically determined structures for NH3 dimer except one. A recent electronic structure calculation... [ABSTRACT FROM AUTHOR]

The symmetry properties of NH3 dimer are analyzed in the molecular symmetry group and the physical assumptions underlying the choice of this group are detailed. Several low lying tunneling states are predicted. Two of these states transform as four-dimensional (G) representations of G36, the molecular symmetry group. These two states can have the same microwave selection rules as those found in the two experimentally observed states of NH3 dimer. That is, the microwave transitions can follow pure rotational selection rules (no tunneling splittings) even though interchange tunneling is included as a feasible motion in the molecular symmetry group. The observation of these selection rules is interpreted as internal rotation stabilizing the system against interchange tunneling. Hence, in these particular states the interchange tunneling is quenched. The group theory predicts this picture of ammonia dimer to be appropriate if certain internal rotation interactions are large compared to the interchange tunneling matrix element. These conclusions are derived through correlation diagrams which are used to provide physical insight into the nature of the two states of G symmetry and approximate electric dipole selection rules for microwave transitions arising from these states. The selection rules for the infrared transitions of this dimer which correlate to the ν2 fundamental of NH3 are also given. [ABSTRACT FROM AUTHOR]

The rotational spectra of C2H4–H2O and C2H4–D2O were measured using the molecular beam electric resonance technique. The rotational and centrifugal distortion constants obtained for C2H4–H2O are: B+C=7274.747 (24), B-C=371.103 (8), A=25 858.4 (36), ΔJ=0.0279 (17), ΔJK=1.7352 (66), and δJ=0.002 99 (22) MHz. The dipole moment for both isotopic species is 1.094 (1) D. The structure derived from an analysis of the rotational constants and dipole moment is nonplanar with Cs symmetry. The water molecule is singly hydrogen bonded perpendicular to the plane of the ethylene; i.e., into the π system. The plane of the water bisects the C–C bond. The hydrogen bond length is 2.48 Å. Splittings are observed in the rotational transitions of C2H4–H2O but not in C2H4–D2O. These are assigned to excited torsional levels of the hindered internal rotation of the water with respect to the ethylene. The barrier height is estimated to be V2=1.0±0.2 kcal/mol which is surprisingly high for this weakly bound complex. [ABSTRACT FROM AUTHOR]

The rotational spectra of NH3–CO, ND3–CO, ND2H–CO, NDH2–CO, NH3–13CO, and NH3–N2 have been measured by molecular beam electric resonance. The K=0 ground vibrational state transitions for these species were fit to a linear molecule Hamiltonian and the following constants were obtained for NH3–CO; (B+C)/2 (MHz)=3485.757(2), DJ (kHz)=110.2(2), eQqNaa (MHz)=-1.890(7), μa (D)=1.2477(8). These constants were also determined forND3–CO [3078.440(7), 75.7(8), -2.028(15), 1.2845(9)], NHD2–CO [3202.303(4), 86.8(6), -1.972(11), 1.2686(8)], NH2D–CO [3338.235(4), 98.9(6), -1.916(12), 1.2546(8)], NH3–13CO [3451.684(5), 108.7(7), -1.870(15), 1.2452(8)]. For NH3–N2 (B+C)/2=3385.76(21), DJ =117.(10), and μa =1.069(14). For NH3–CO three |ΔJ|=1, K=0 progressions were seen along with two |ΔJ|=1, K=1 progressions, suggesting nonrigidity in the complex. The internal rotation of the NH3 subunit about its C3 axis is expected to be essentially free, but this motion, by itself, is not sufficient to explainthe observed spectra, thus, large amplitude dynamics are occurring in at least two degrees of freedom. The quadrupole coupling constants, eQqNaa indicate that in each of the isotopes of NH3–CO the NH3 subunit has its C3 axis relatively rigidly oriented at an angle of approximately 36° with respect to the line connecting the centers of mass of the two subunits. The structure is not hydrogen bonded; the N atom is closest to the CO subunit. The orientation of the CO subunit is not established. The distance between the N atom and the center of mass of the CO unit (RN–CO) is 3.54(3) Å. The spectroscopic constants suggest that the weak bond stretching force constant is quite small (0.01 mdyn/Å) but compatible with the long bond length. [ABSTRACT FROM AUTHOR]

High resolution rotational spectra for (14NH3)2, (14ND3)2, 15NH3–14NH3, and 14NH3–15NH3 have been obtained using the molecular beam electric resonance technique. For 14NH3 dimer the spectra are simple and consist only of the J=0–1 and J=1–2 transitions with K=0 for two vibrational states (hereafter designated α and β) of the complex. The spectroscopic constants for (14NH3)2 are: 1/2 (B+C)α=5110.412(2), 1/2 (B+C)β=5110.564(2), DJ=0.0529(5), eqQ1aa =-0.639(13), eqQ2aa =0.904(8) (MHz); μαa =0.7497(10), μ βa =0.7376(9) (D). For (ND3)2 the vibrational states are unsplit 1/2 (B+C)=4190.300(50) MHz and μa=0.567(4) D. These spectroscopic constants are inconsistent with the theoretically predicted structure for NH3 dimer. In the predicted structure one NH3 unit hydrogen bonds to the other with a nearly linear N–H- –N arrangement so that the C3 axis of the basic NH3 is collinear with the hydrogen bond. If θi is the angle between the C3 axis of the ith NH3 unit and the a-inertial axis of the complex, θ1∼0° and θ2∼68° forthe theoretically predicted structure. In contrast, for 14NH3 dimer we measure the average values of θ1 and θ2 as 48.6(1)° and 115.5(1)°, respectively, using the quadrupole hyperfine structure. Furthermore, the component of the dipole moment along the a-inertial axis, μa, is found to be much smaller (0.75 D) than expected for the theoretical structure (∼2 D). The distance between the centers of mass of the NH3 subunits, RCM, is also determined and is found to be 3.3374(1) Å. No evidence exists for nonrigidity in the ground vibrational state of this complex, i.e., no high frequency tunneling splittings were observed in either the microwave or the infrared spectrum of the complex. The relationship between these structural results and the concept of hydrogen bonding in NH3 systems is discussed. The... [ABSTRACT FROM AUTHOR]

The rotational spectrum of NH3–N2O has been observed by the molecular beam electric resonance method. The spectrum is characteristic of a T-shaped complex in which the nitrogen of the NH3 subunit is directed toward the N2O subunit. The NH3 unit exhibits nearly free internal rotation about its C3 axis. The A internal rotor state transitions were fit to Watson’s asymmetrical top Hamiltonian and the following spectroscopic constants were determined: A(MHz)=12 722.5(5), δK (MHz)=0.22(2), B(MHz)=4 083.5(2), δJ(MHz) =0.007(1), C(MHz)=3 070.8(2), ΔK(MHz) =-0.3(2), ΔJ(MHz) =0.016(9), ΔJK(MHz) =0.32(3), μa(D) =1.514(9), μb(D) =0.09(9). The E internal rotor state transitions were fit to the Hamiltonian of Kilb, Lin, and Wilson. For the E internal rotor state the rotational constants A [12 743(64) MHz], B[4077.1(12) MHz], and C[3069.9(9) MHz] were determined, as well as the height of the barrier to internal rotation, V3 [12.5(25) cm-1]. The distance between the nitrogen of the NH3 to the center of mass of N2O is 3.088 Å. The angle between the C3 axis of NH3 and the line joining the centers of mass of the NH3 and N2O subunits is 13°. The C3 axis of NH3 is pointed towards the nitrogen end of the N2O subunit. [ABSTRACT FROM AUTHOR]

The radio frequency and microwave spectrum of (SO2)2 has been measured using the molecular beam electric resonance technique. The spectrum is characteristic of an asymmetric top in which the two nonequivalent SO2 subunits interchange roles through a low frequency (70 kHz) tunneling motion. The spectroscopic constants obtained for SO2 dimer are: (B+C/2) (MHz) 926.160(2), (B-C/2) (MHz) 22.3207(1), A-(B+C/2) (MHz) 6032.3(6), ΔJ(MHz) 0.00217(2), ΔJK(MHz) 0.0995(1), δinv(MHz) 0.070(1), μa(D) 1.4052(13). The average distance between the centers of mass of the two subunits, RCM, is 3.825(10) Å. The magnitude of the weak bond stretching force constant ks is 0.0264(4) mdyn/Å. The relative orientation of the subunits is not well determined, but is demonstrated to be unlike the orientation of the nearest neighbors in the sulfur dioxide crystal. [ABSTRACT FROM AUTHOR]

We present the results of microwave and infrared spectroscopic studies of several van der Waals complexes of NH3. These results were obtained with a molecular beam electric resonance spectrometer. The microwave spectroscopy of the complexes (NH3)2 and Ar–NH3 show that both systems are nonrigid. The observed dipole moments for (NH3)2[0.74(2) D] and (ND3)2[0.57(1) D] are not compatible with the presently accepted theoretical structure. Ar–NH3, which has a complicated and currently unassigned microwave spectrum, exhibits Q branch inversion transitions near 19 GHz which indicate that the NH3 subunit is likely to be a near-free rotor. Infrared studies of the complexes NH3–HCCH, NH3–CO2, (NH3)2, Ar–NH3, NH3–OCS, NH3–N2O, and NH3–HCN have been carried out with a line tunable CO2 laser. Only for NH3–HCN were no infrared resonances discovered. Photodissociative transitions are observed in all of the other systems. Band origins for the photodissociative infrared transitions involving the ν2 umbrella motion of NH3 were determined for NH3–HCCH [984.4(9) cm-1], NH3–CO2 [987.1(2) cm-1]. NH3–OCS [981.5(15) cm-1], and NH3–N2O [980(2) cm-1]. The observation of an infrared transition for Ar–NH3 at 938.69 cm-1, which is 40 cm-1 lower than the band origins in the other NH3 complexes, lends support to the model of Ar–NH3 mentioned above. NH3–HCCH, NH3–CO2, (NH3)2, and Ar–NH3 were studied in microwave-infrared double resonance experiments in order to eliminate much of the inhomogeneous broadening present in their infrared spectra and to aid in the rotational assignment of the infrared spectra. Linewidths were determined for NH3–HCCH (0.15 GHz) and for NH3–CO2 [14(6) GHz]. An important result of this study is that the dissociation energies of all the complexes studied, except for NH3–HCN, are established to be less than 990 cm-1, i.e.... [ABSTRACT FROM AUTHOR]