Kenneth R. Leopold

Tunable far‐infrared spectroscopy of malonaldehyde

A tunable far‐infrared difference frequency spectrometer has been used to examine the fully protonated form of malonaldehyde in the region near the ground‐state tunneling frequency (21 cm−1). An extremely dense and complex spectrum is observed in which the strongest features have been assigned as pure rotational lines involving high values of J and K−1. These transitions, which occur within the individual rotational manifolds of the two halves of the ground‐state tunneling doublet, have been analyzed simultaneously with existing microwave data for this species. The value of the ground‐state tunneling splitting, determinable indirectly from analysis of vibration–rotation interactions, is 21.584 76(17) cm−1, and is in close agreement with that similarly obtained in previous microwave work. A thorough treatment of the centrifugal distortion in this system significantly extends the range of rotational states whose energies may be reliably calculated, and should therefore be valuable in the future direct measu...

Observation of three intermolecular vibrational states of ArHF

Abstract We report the observation of the (0, 1 1 , 0), (0, 2 0 , 0), and (0, 0 0 , 2) states of the weakly bound complex ArHF by direct far infrared absorption spectroscopy in a jet. A simultaneous analysis of all observed rovibrational frequencies has been conducted, and spectroscopic constants sensitive to the global topology of the intermolecular potential energy surface are given.

Determination of the dipole moment of H3N–SO3 in the gas phase

Abstract The dipole moment of the partially bonded molecule H 3 N–SO 3 has been measured by microwave spectroscopy in the gas phase. The value obtained, 6.204(11) D, is over 4 times the sum of the individual monomer moments. The results are discussed in the context of recent speculations as to the complexs role in the nucleation of atmospheric aerosols.

Far infrared difference frequency spectroscopy of the weak bond in ArHBr

Abstract A new tunable far infrared difference frequency spectrometer has been used to observe the Q branch of the lowest Π bending vibration of ArH 79 Br and ArH 81 Br. The vibrational frequencies are 26.665635(4) and 26.665284(4) cm -1 for the 79 Br and 81 Br species respectively. The average center-of-mass separation is 0.07 A shorter in the excited state than in the ground state and analysis of the bromine nuclear hyperfine structure provides the excited state angular expectation value 〈 P 2 (cosθ)〉 = -0.098(2). The data indicate significant coupling between the bending and stretching coordinates of the complex at angles sampled by the Π state. The results are compared with the best available theoretical calculations for this species and the agreement is found to be excellent.

Determination of the threefold internal rotation barrier in ArNH3

The two Σ and four Π states of the weakly bound complex Ar–NH3 correlating to j=2, k=±1 ammonia have been observed by tunable far infrared difference frequency‐microwave sideband spectroscopy. The results have been combined with published data to determine a new angular potential energy surface for the system. The barrier to threefold internal rotation of the NH3 about its C3 axis in the complex is estimated to be 25.606(24) cm−1 near the minimum energy (T‐shaped) configuration. The potential also exhibits maxima at both symmetric top configurations, with energies approximately 53 and 31 cm−1, respectively above that of the global minimum. The location and splitting between the symmetric and antisymmetric Σ states are indicative of a strong interaction with another pair of unobserved states, most likely the first excited intermolecular stretch built on j=1, k=±1 Ar–NH3.

The Formic Acid–Nitric Acid Complex: Microwave Spectrum, Structure, and Proton Transfer

Rotational spectra are reported for seven isotopologues of the complex HCOOH-HNO3 in a supersonic jet. The system is planar and bound by a pair of hydrogen bonds, much like the more widely studied carboxylic acid dimers. Double proton exchange interconverts the system between a pair of equivalent structures, as revealed by a splitting of the a-type spectrum that disappears when one of the hydrogen bonding protons is replaced by deuterium. The observation of relative intensities that are consistent with nuclear spin statistics in a symmetric and antisymmetric pair of tunneling states provides additional evidence for such a motion. The observed splittings in the pure rotational spectrum are 1-2 orders of magnitude smaller than those recently reported in the pure rotational spectra of several related carboxylic acid dimers. This is a curious difference, although we note that because the observed spectra do not cross the tunneling doublet, the splittings are a measure of the difference in effective rotational constants for the two states, not the tunneling frequency itself. The observed rotational constants have been used to determine an accurate vibrationally averaged structure for the complex. The two hydrogen bond lengths, 1.686(17) Å and 1.813(10) Å for the hydrogen bonds involving the HNO3 and HCOOH protons, respectively, differ by 0.127(27) Å. Likewise, the associated oxygen-oxygen distances determined for the parent species, 2.631 and 2.794 Å, differ by 0.163 Å. These results suggest that the double proton transfer is necessarily accompanied by substantial motion of the heavy atom frame, and thus this system, in principle, provides an excellent prototype for multidimensional tunneling processes. Ab initio calculations of the binding energy and the barrier height are presented. Excellent agreement between the calculated equilibrium structure and the experimental, vibrationally averaged structure suggests that the vibrational wave function is not highly delocalized in the region between the equivalent potential wells. (14)N nuclear quadrupole hyperfine structure is interpreted in terms of the degree to which the HNO3 releases its proton in either of the equivalent potential energy minima.

Partial Proton Transfer in the Nitric Acid Trihydrate Complex

Four isotopologues of the gas-phase complex HNO(3)-(H(2)O)(3) have been observed by microwave spectroscopy in a supersonic jet. Rotational and nuclear electric quadrupole coupling constants have been obtained and the experimentally derived inertial defect has been used to infer a near-planar geometry for the complex. The data identify the observed species from among several structures predicted by theory, favoring a 10-membered ring geometry with the HNO(3) hydrogen-bonded to the first water, a series of water-water hydrogen bonds, and ring completion with the third water acting as a hydrogen-bond donor to an unprotonated HNO(3) oxygen. This structure corresponds to the lowest energy form predicted computationally in several prior studies as well as in this work using the MP2/6-311++G(2df,2pd) level/basis set. Although its observation does not rigorously establish its status as the lowest energy form, the concurrence between the predicted low-energy conformer and that observed in the ultracold supersonic jet strongly suggests that it is indeed the minimum-energy structure. The a-type spectra show evidence of internal dynamics, likely resulting from large amplitude motion of one or more of the water subunits. This complex represents the third step in the sequential hydration of HNO(3), and both the theoretical structure and experimental (14)N quadrupole coupling constants have been used to track the degree of ionization of the acid as function of hydration number. Based on (14)N quadrupole coupling constants, transfer of the HNO(3) proton to its nearest water molecule is about one-third complete in the trihydrate.

Gas phase observation and microwave spectroscopic characterization of formic sulfuric anhydride

An unexpected gaseous sulfur species Sulfuric acid plays a central role in both industrial and atmospheric contexts. As such, the behavior of SO3 mixtures in gas phases has been studied for over a century. In gas-phase experiments on wet SO3 and formic acid, Mackenzie et al. discovered a previously unrecognized covalent adduct: formic sulfuric anhydride, or HC(O)OSO3H. The combination of microwave spectroscopy and theoretical calculations reveals its structural properties. The compound may play a role in the nucleation of atmospheric aerosols by serving as an intermediate to H2SO4 formation. Science, this issue p. 58 An unexpected covalent adduct of formic acid and sulfur trioxide has been characterized in the gas phase. We report the observation of a covalently bound species, formic sulfuric anhydride (FSA), that is produced from formic acid and sulfur trioxide under supersonic jet conditions. FSA has been structurally characterized by means of microwave spectroscopy and further investigated by using density functional theory and ab initio calculations. Theory indicates that a π2 + π2 + σ2 cycloaddition reaction between SO3 and HCOOH is a plausible pathway to FSA formation and that such a mechanism would be effectively barrierless. We speculate on the possible role that FSA may play in the Earth’s atmosphere.

Coriolis coupling in ArHCl

Abstract The Π bending state of ArHCl has been examined by tunable far-infrared spectroscopy and the J dependence of the Coriolis coupling to the Σ bending and Σ stretching vibrations analyzed. The Coriolis coupling constants obtained are only slightly smaller than those previously observed in the complex containing a quantum of HCl of vibrational excitation (3.2% for the Π—Σ bend coupling and 4.5% for the Π—Σ stretch coupling). Deperturbed rotational constants for the Σ bending state of the 35 Cl and 37 Cl species cannot be used in conjunction with simple moment of inertia expressions to support the predicted antihydrogen bonded geometry for that state. Comparisons between the ground states and the Π states of the series ArHX (Xz.dbnd;F, Cl, Br) are discussed.

3D-printed slit nozzles for Fourier transform microwave spectroscopy

3D printing is a new technology whose applications are only beginning to be explored. In this report, we describe the application of 3D printing to the design and construction of supersonic nozzles. Nozzles can be created for