Molecular Beam Electric Resonance Spectroscopy Research Articles

Characterization of Oriented Molecule Beams by Radio Frequency Spectroscopy

Molecular-beam electric-resonance spectroscopy is used to interrogate the rotational states present in a molecular beam of oriented symmetric-top molecules produced for scattering experiments. ¢M )( 1 transitions are observed between Stark energy levels in weak electric fields and depend on the rotational quantum numbers J and K. Substantial rotational cooling is apparent in both neat and seeded beams. Each resonance signal has a complicated dependence upon the high voltage applied to the hexapole focusing fields because molecules in the newly transformed states have vastly different focusing properties from the original. These effects can be unified using a “reduced” focusing voltage that allows intensity comparisons between rotational states, giving rotational temperatures of 3-4 K for CF3H seeded in He or Ar. Under favorable circumstances, radio frequency “labeling” might allow one to selectively remove one rotational level at a time from an oriented molecular beam and thereby to study the orientation dependence of different rotational states.

Sub-Doppler Infrared Spectra and Torsion–Rotation Energy Manifold of Methanol in the CH-Stretch Fundamental Region

The infrared spectrum of jet-cooled methanol in the CH-stretch fundamental region has been investigated by two sub-Doppler laser techniques: optothermally detected molecular-beam electric resonance and direct-absorption slit-jet spectroscopy. With the aid of microwave-infrared double resonance and ground state combination differences, 27 subbands in the frequency range 2967 to 3027 cm−1have been assigned to the ν2fundamental. Perturbation systems in theK′ = 0E, −1E, and −2Esymmetry subbands have been analyzed to yield matrix elements of 0.013, 0.041, and 0.75 cm−1, respectively. TheA–Etorsional tunneling splitting forJ= 0 of the ν2vibration of −3.26 cm−1is of opposite sign and a factor of three smaller in magnitude than the ground state value of +9.12 cm−1.

Microwave determination of the structure of the Cs conformation of dipropyl ether

Microwave spectroscopy of dipropyl ether has been performed by supersonic jet molecular beam electric resonance spectroscopy and by molecular beam pulsed-jet Fourier transform spectroscopy. Calculations suggest that there are four distinct conformations of dipropyl ether that have energies within 0.5 kcal/mol of each other and have geometries of C 2υ , C 2 , C 3 , and C 1 symmetry. The geometry of the C s conformation has been determined. The rotational constants of this conformation are A=4793.6393(5) MHz, B=1242.5098(3) MHz, C=1053.3520(2) MHz, δ J =0.117(6) kHz, δ K =1.25(5) kHz, Δ JK =-2.807(7) kHz, Δ J =0.448(2) kHz, and Δ K =11.57(3) kHz

The structure of N2⋅SO2 from diode laser and molecular-beam electric resonance spectroscopy

The b-type vibration–rotation band of N2⋅SO2 near the SO2 ν3 band origin was observed in a molecular-beam, diode laser direct absorption experiment. Rotational transitions and Stark effect data for this complex were additionally measured using molecular-beam electric resonance methods. The vibrational band origin was 1361.1440(2) cm−1, shifted by 0.9167(2) cm−1 from that of the SO2 monomer. Rotational constants were measured for the upper and lower vibrational states with A″=8875.3(22) MHz, B″=1620.3(22) MHz, C″=1426.1(24) MHz, A′=8832.4(26) MHz, B′=1617.3(28) MHz, and C′=1431.6(15) MHz. The electric dipole moment components were determined, with μa = 0.0441(16) D and μc = 1.5884(29) D. The c component of the nitrogen quadrupole coupling component was found to be eqccQ = 1.30(21) MHz. A structure analysis gave the separation between the centers of mass of the monomers as 3.8925(28) Å. The angles between the symmetry axes of the SO2 and N2 units and the line connecting these monomers were calculated as 61.35° and 24.54°, respectively. Additionally, the SO2 monomer a axis was found to lie along the b axis of the complex. The electric dipole moment data indicate that the equilibrium angle for the SO2 is much closer to 90° than the rms result. These structural results were compared to model calculations of the binding energy of the complex.

The rotational spectrum, internal rotation, and structure of H2O–NCCN and D2O–NCCN

The radio frequency and microwave spectra of H2O–NCCN and D2O–NCCN have been measured using molecular beam electric resonance spectroscopy. The spectrum is characteristic of a planar T-shaped asymmetric top in which the H2O subunit exhibits hindered twofold internal rotation. The spectroscopic parameters for H2O–NCCN are A=4692.11(2) MHz, A′=4689.45(25) MHz, B=4258.06(1) MHz, C=2219.34(1) MHz, and μa =2.06(3) D and for D2O–NCCN are A=4644.31(27) MHz, B=3822.76(10) MHz, C=2084.86(10) MHz, and μa =2.1617(10) D. The H2O subunit is bound to NCCN through the oxygen atom with a twofold barrier of 285(6) cm−1 hindering the internal rotation of H2O about the a axis of the complex.

The dipole moment of water. I. Dipole moments and hyperfine properties of H2O and HDO in the ground and excited vibrational states

Molecular beam electric resonance spectroscopy has been used to study H2O in its ground state and in each of its singly excited vibrational states. HDO was studied in its ground state and first excited O–H stretching state. Precise dipole moments have been obtained for a total of 14 rotational levels of six vibrational states. Centrifugal distortion corrections to the Stark coefficients were made and rotationless moments were calculated for each vibrational state. The magnitude of the H2O moment can be written μ(v1v2v3) =1.857 04+0.005 08(v1+ (1)/(2) ) −0.031 66(v2+ (1)/(2) ) +0.022 46(v3+ (1)/(2) ). Individual components of the HDO moment were obtained for the ground and excited states. Hyperfine properties were also determined for each of the vibrational states studied. In the following paper, the dipole moments are combined with infrared transition moments to obtain an improved dipole moment function for water.

Preliminary structural characterization of complexes of cyanogen: NH3-NCCN and (NCCN)2

The complexes of NCCN with itself and with NH 3 have been studied by molecular beam electric resonance spectroscopy. In both species a heavy-atom arrangement of C 2v symmetry is observed. The NH 3 -NCCN complex appears to exhibit essentially free internal rotation of the NH 3 subunit about its internal symmetry axis

Electric dipole moment and hyperfine properties of bromoacetylene in the ground and first excited C–H stretching vibrational states

Molecular beam electric resonance spectroscopy, combined with color center laser excitation, has been used to measure the electric dipole moment and Br hyperfine properties of bromoacetylene in its ground and first excited C–H stretching vibrational states. For the HCC79Br, v=0: μ=0.229 62(1) D, eQq=648.113(3) MHz, CBr=6.4(3) kHz, and B=4000.07 MHz. For the HCC79Br, v=1: μ=0.248 82(1) D, eQq=648.160(2) MHz, CBr=7.0(3) kHz, and B=3992.79 MHz. For the HCC81Br, v=0: μ=0.229 56(1) D, eQq=541.430(1) MHz, CBr=7.4(1) kHz, and B=3978.46 MHz. For the HCC81Br, v=1: μ=0.248 61(1) D, eQq=541.464(3) MHz, CBr=7.6(4) kHz, and B=3971.34 MHz. The vibrational band origin for the C–H stretching vibration is 3367.7(2) cm−1.

Radio frequency and microwave spectroscopy of the HCCH–CO2 and DCCD–CO2 van der Waals complexes

Molecular beam electric resonance spectroscopy has been used to study HCCH–CO2 and DCCD–CO2, giving the following results for HCCH–CO2: A=8875 MHz, B=2861.45 MHz, C=2156.25 MHz, DJ =12.4 kHz, DJK =36.7 kHz, and d1=−3.25 kHz. The permanent dipole moment is 0.1611 D. The equilibrium geometry has the monomers parallel to one another in a configuration having C2v symmetry. Deuterium eQq data provide information on rms vibrational amplitudes and also gives an estimate of 40 cm−1 for the out of plane bending mode. The monomer–monomer stretching vibration is estimated to be 75 cm−1 from the DJ measurement.

Radiofrequency spectroscopy of DCCD: Deuterium quadrupole coupling in acetylene

Molecular beam electric resonance spectroscopy has been used to observe the P(16) e and P(16) f transitions in the ν 5- ν 4 band of dideutero acetylene. Hyperfine transitions were resolved in the P(16) e case, providing deuterium quadrupole coupling and spin rotation information. The hyperfine properties are essentially the same for ν 4, J = 16 and ν 5, J = 15 with eQq = 209(1) kHz and C = −0.42(2) kHz. This information is compared with other measurements of deuterium quadrupole coupling in acetylene.

Stark, Zeeman, and hyperfine properties of v=0, v=1, and the equilibrium configuration of hydrogen fluoride

Molecular beam electric resonance spectroscopy has been done on the v=0 and v=1 states of hydrogen fluoride in external electric and magnetic fields. The v=1 state was populated using a color center laser. v=0, v=1, and equilibrium configuration results have been obtained for the HF dipole moment, fluorine and proton spin rotation interactions, fluorine and proton magnetic shielding anisotropies, both direct and indirect spin–spin interactions, rotational magnetic moment, and magnetic susceptibility. All of these properties can be extrapolated to v=2 with high accuracy and to higher states with reasonable confidence. First and second derivatives with respect to internuclear distance have also been obtained for several of these properties. Comparisons of HF and DF dipole moments indicate significant differences arising from Born–Oppenheimer breakdown.

The microwave spectrum of the K=0 states of Ar–NH3

The microwave spectrum of Ar–NH3 has been obtained using molecular beam electric resonance spectroscopy and pulsed nozzle Fourier transform microwave spectroscopy. The spectrum is complicated by nonrigidity and most of the transitions are not yet assigned. A ΔJ=1, K=0 progression is assigned, however, and from it the following spectroscopic constants are obtained for Ar–14NH3: (B+C)/2=2876.849(2) MHz, DJ =0.0887(2) MHz, eqQaa =0.350(8) MHz, and μa =0.2803(3) D. For Ar–15NH3 we obtain (B+C)/2 =2768.701(1) MHz and DJ =0.0822(1) MHz. The distance between the Ar atom and the 14NH3 center of mass RCM is calculated in the free internal rotor limit and obtained as 3.8358 Å. In the pseudodiatomic approximation, the weak bond stretching force constant is 0.0084 mdyn/Å which corresponds to a weak bond stretching frequency of 35 cm−1. The NH3 orientation in the complex is discussed primarily on the basis of the measured dipole moment projection and the quadrupole coupling constant. It is concluded that the Ar–NH3 intermolecular potential is nearly isotropic and that the NH3 subunit undergoes practically free internal rotation in each of its angular degrees of freedom. Spectroscopic evidence is presented which indicates that the NH3 subunit also inverts within the complex. These conclusions concerning the internal dynamics in the Ar–NH3 complex support the model initially proposed in our previous study of the microwave and infrared spectra of this species.

Determination of the structure of HCl BF3

The structure of the weakly bound complex HCl BF3 has been determined by molecular beam electric resonance spectroscopy. The molecule is a near prolate symmetric top with the chlorine atom 3.17 Å above the boron atom, on or very near the BF3 threefold axis. The B–Cl–H angle is close to 90°. The spectroscopic constants for the H35Cl11BF3 isotope are (B+C)/2=1774.117(4) MHz, DJ=5.6(5) kHz, eqQa(35Cl)=+25.761(8) MHz, eqQa(11B)=+2.672(25) MHz, and μa=0.484(5)D.

ChemInform Abstract: Rotational Spectra and Structure of the Ammonia‐Water Complex.

AbstractMikrowellen‐ und Hochfrequenzspektren für NH3 ‐ H2O sowie deuterierte Analoga werden mit Hilfe der Molekularstrahlresonanzmethode aufgenommen und Rotationskonstanten, Stark‐Effekt sowie Daten für die Stickstoff‐Quadrupolko‐pplungswechselwirkung ermittelt.

Rotational spectra and structure of the ammonia–water complex

Microwave and radio frequency spectra for NH3⋅H2O and deuterated analogs have been observed by molecular beam electric resonance spectroscopy. Rotational constant, Stark effect, and nitrogen quadrupole coupling interaction data were obtained. This complex is found to have a linear, hydrogen bonded structure with water as the proton donor. The NH3 monomer symmetry axis was found to have a vibrationally averaged displacement of 23.1° from the N...O axis. No evidence for transfer of a proton from water to the ammonia was observed.

Molecular beam electric resonance study of the ground and excited states of cyanoacetylene

Cyanoacetylene was studied using molecular beam electric resonance spectroscopy. Ground state results are μ=3.731 72 D, eQq=−4319.24 kHz, CN=0.976 kHz. Zero field spectra were observed for the three singly excited bending modes and Stark data were taken for the lower energy of two of these three states. Excited state results include ν7; μ=3.7225 D, eQqaa=−4302.0 kHz, eQ(qbb−qcc)=−28.8 kHz, CN(J=2)=1.47 kHz; ν6; μ=3.7263 D, eQqaa=−4369.7 kHz, eQ(qbb−qcc)=177.1 kHz, CB(J=2)=1.30 kHz; ν5; eQqaa=−4325.7, eQ(qbb−qcc)=96.8 kHz, CN(J=2)=1.33 kHz. l-doubling constants were also obtained for the degenerate vibrations.

The microwave and radiofrequency spectrum of H2S⋅Ar

Molecular beam electric resonance spectroscopy has been used to determine the radiofrequency and microwave spectrum of H2S⋅Ar, HDS⋅Ar, and D2S⋅Ar. The electric dipole moment and nuclear hyperfine interactions reflect an angular structure in which the H2S C2 axis is nearly perpendicular to the Ar...S axis, and the H2S and argon are nearly coplanar. The effective argon–H2S center-of-mass distance is 3.977 Å.

Microwave and radiofrequency Stark spectrum of ArHCN: A highly nonrigid molecule

The rotational spectrum of the weakly bound complex ArHCN has been observed using molecular beam electric resonance spectroscopy. The spectrum is superficially characteristic of that of a linear molecule with both unusually large centrifugal distortion (requiring a J6 dependent distortion term to fit the data) and an unexpectedly large bending amplitude. The spectroscopic constants are The centrifugal distortion constant DJ is remarkably large and abnormally sensitive to isotopic substitution. Using the usual model, the stretching and bending force constants obtained from these data are an order of magnitude smaller than those similarly computed for the hydrogen halide complexes of argon. The calculated stretching and bending frequencies are 10 cm−1, predicting that excited vibrational levels should be populated in the beam. Three transitions have been observed which appear to correspond to an excited vibrational level of ArDCN, but poor signal-to-noise has prohibited their unambiguous assignment. While seemingly, the data support a hydrogen bonded structure for this complex, they require an anomalously short bond length of 2.7 A, despite the apparent weakness of the bond. The vibrationally averaged molecular geometry and force constants are seen to be unexpectedly sensitive to isotopic substitution. Our conclusion is that the observed behavior of this system is best interpreted in terms of strong coupling between the radial and angular degrees of freedom associated with the weak bond. The amplitude of oscillation in the two coordinates is large, making the ‘‘structure’’ of this complex nebulous.

Water–hydrocarbon interactions: Rotational spectroscopy and structure of the water–acetylene complex

The radiofrequency and microwave spectra of C2H2–H2O, C2H2–D2O, C2D2–H2O, and C2D2–D2O have been measured by molecular beam electric resonance spectroscopy. Rotational constants and dipole moments are reported. The structure is effectively planar with the acetylene hydrogen bonded to the oxygen of the water. The hydrogen-bond length and stretching force constant are calculated to be. 2.229 Å and 0.065 mdyn/Å, respectively. Considering the lone pair orbital structure of the water molecule, an equilibrium structure having the water plane tilted away from the a axis of the complex is expected. Even so, a comparison of the dipole moments for the different isotopically substituted species shows that the height of the barrier hindering an inversion motion is low enough that no vibrational levels lie below the top of the barrier in the double-minimum potential well.

Molecular beam electric resonance study of KCN, K 13CN and KC 15N

The microwave spectra of the isotopic species K 13CN and KC 15N have been investigated by molecular beam electric resonance spectroscopy, using the seeded beam technique. For both isotopic species about 20 rotational transitions originating in the ground vibrational state were observed in the frequency range 9–38 GHz. The observed transitions were fitted to an asymmetric rotor model to determine the three rotational, as well as the five quartic and three sextic centrifugal distortion constants. The hyperfine spectrum of KCN has been unravelled with the help of microwave-microwave double-resonance techniques. One hundred and forty hyperfine transitions in 11 rotational transitions have been assigned. The hyperfine structures of K 13CN and KC 15N were also studied. For all three isotopic species the quadrupole coupling constants and some spin-rotation coupling constants could be deduced. The rotational constants of the 13C and 15N isotopically substituted species of potassium cyanide, combined with those of the normal isotopic species (determined more accurately in this work), allowed an accurate and unambiguous evaluation of the structure, which was confirmed to be T shaped. Both the effective structure of the ground vibrational state and the substitution structure were evaluated. The results for the effective structural parameters are r CN = 1.169(3) A ̊ , r KC = 2.716(9) A ̊ , and r KN = 2.549(9) A ̊ . The values obtained for the principal hyperfine coupling constant eQq z (N), the angle between the CN axis and z N, and the bond length r CN indicate that in gaseous potassium cyanide the CN group can be considered as an almost unperturbed CN − ion.