Slit Jet Expansion Research Articles

Infrared spectra of acetylene dimers and acetylene–nitrogen: (DCCD) 2, H-bonded DCCD–HCCH, and DCCD–NN in the 4.1 μm region

Infrared spectra of the weakly-bound T-shaped acetylene dimers DCCD–DCCD and DCCD–HCCH are studied in the region of the DCCD ν 3 fundamental (∼2440 cm −1) using a pulsed supersonic slit-jet expansion and a tunable diode laser probe. The K a = 0 ← 1 and 1 ← 0 subbands, corresponding to the vibration of the DCCD monomer at the “top” of the T, are analyzed. Compared to the analogous spectrum of HCCH–HCCH, the present results are much less perturbed. The tunneling splitting for (DCCD) 2 in the excited state is determined to be 141 MHz, a big reduction from the previously determined ground state value of 424 MHz. The dimer A rotational constants show a large apparent increase upon vibrational excitation, and we discuss whether this increase is real. The linear DCCD–NN complex is also observed as an impurity in the spectrum, and it too is found to be unperturbed, in contrast with HCCH–NN.

Spectroscopic observation and structure of CS2 dimer

Infrared spectra of the CS(2) dimer are observed in the region of the CS(2) ν(3) fundamental band (∼1535 cm(-1)) using a tunable diode laser spectrometer. The weakly bound complex is formed in a pulsed supersonic slit-jet expansion of a dilute gas mixture of carbon disulfide in helium. Contrary to the planar slipped-parallel geometry previously observed for (CO(2))(2), (N(2)O)(2), and (OCS)(2), the CS(2) dimer exhibits a cross-shaped structure with D(2d) symmetry. Two bands were observed and analyzed: the fundamental (C-S asymmetric stretch) and a combination involving this mode plus an intermolecular vibration. In both cases, the rotational structure corresponds to a perpendicular (ΔK = ±1) band of a symmetric rotor molecule. The intermolecular center of mass separation (C-C distance) is determined to be 3.539(7) Å. Thanks to symmetry, this is the only parameter required to characterize the structure, if the monomer geometry is assumed to remain unchanged in the dimer. From the band centers of the fundamental and combination band an intermolecular frequency of 10.96 cm(-1) is obtained, which we assign as the torsional bending mode. This constitutes the first high resolution spectroscopic investigation of CS(2) dimer.

The weakly-bound CO 2–acetylene complex: Fundamental and torsional combination band in the CO 2ν3 region

Infrared spectrum of the weakly-bound CO 2–C 2H 2 complex in the region of the CO 2 ν 3 fundamental band (∼2349 cm −1) is observed in a pulsed supersonic slit jet expansion using a tunable diode laser probe. Two bands are observed and analyzed: the fundamental (C–O asymmetric stretch) and a combination involving the intermolecular torsional (out-of-plane bend) vibration. The resulting torsional frequency is 44.385(10) cm −1. This represents the first observation of an intermolecular frequency for carbon dioxide–acetylene complex. A comparison between the results obtained here and those previously reported for N 2O–C 2H 2 complex is discussed.

Infrared spectrum of the CS2 trimer: observation of a structure with D3 symmetry

Infrared spectra of a carbon disulfide trimer formed in a pulsed supersonic slit-jet expansion are obtained via direct absorption of a tuneable diode laser in the region of the CS(2)ν(3) fundamental (∼1535 cm(-1)). This is the first high-resolution spectroscopic observation of (CS(2))(3). Two bands sharing the same lower state are assigned to ((12)C(32)S(2))(3). These correspond to the two infrared active trimer vibrations (a parallel and a perpendicular band) of the constituent CS(2) monomer asymmetric stretches. The weaker perpendicular band is centered at 1524.613 cm(-1), shifted by -10.74 cm(-1) with respect to the free CS(2) monomer. The parallel band is centered at 1545.669 cm(-1), a vibrational shift of +10.31 cm(-1). Transitions with K≠ 3n and those with K = 0, J = odd in the ground state are absent, establishing that this trimer has D(3) symmetry. The two parameters required to define this structure are determined to be 3.811 Å for the C-C bond distance and 61.8° for the angle between a monomer axis and the plane containing the C atoms. In addition, a parallel band arising from trimers with a single (34)S substitution is observed around 1544.46 cm(-1). Together with the recently observed cross-shaped CS(2) dimer, these results indicate a tendency for CS(2) to form highly symmetric clusters.

Electronic Spectra of C 6H + and C 6H 3+ in the Gas Phase

Measurement of the (3)Pi-(3)Pi transition of C(6)H(+) in the gas phase near 19486 cm(-1) is reported. The experiment was carried out with a supersonic slit-jet expansion discharge using cavity ringdown absorption spectroscopy. Partly resolved P lines and observation of band heads permitted a rotational contour fit. Spectroscopic constants in the ground and excited-state were determined. The density of ions being sampled is merely 2 x 10(8) cm(-3). Broadening of the spectral lines indicates the excited-state lifetime to be approximately 100 ps. The electronic transition of HC(6)H(2)(+) at 26402 cm(-1) assumed to be (1)A(1)-X (1)A(1) in C(2v) symmetry could not be rotationally resolved.

High-resolution cavity ringdown spectroscopy of the jet-cooled propyl peroxy radical C3H7O2

We have obtained high resolution, partially rotationally resolved, jet-cooled cavity ringdown spectra of the origin band of the A<--X electronic transition of two of the five conformers (G(1)G(2) and G(1)T(2)) of the normal propyl peroxy radical, C(3)H(7)O(2), as well as the G conformer of the iso-propyl peroxy radical isomer. This transition, located in the near infrared, was studied using a narrow band laser source (less than or approximately 250 MHz) and a supersonic slit-jet expansion coupled with an electric discharge allowing us to obtain rotational temperatures of about 15 K. All three spectra have been successfully fitted using an evolutionary algorithm approach with a Hamiltonian including rotational and spin-rotational terms. Excellent agreement with the experimental spectra was obtained by fitting seven molecular parameters in each of the ground and the first excited electronic states as well as the band origin of the electronic transition. These parameters are compared with the results from electronic structure calculations. This analysis confirms unambiguously the previous room-temperature conformer assignments that were based upon quantum chemistry calculations.

Direct absorption spectroscopy of water clusters formed in a continuous slit nozzle expansion

In this article, we report on a Fourier transform infrared study of absorption bands belonging to small-sized water clusters formed in a continuous slit nozzle expansion of water vapor seeded in argon carrier gas. Clear signatures of free and H-bonded OH vibrations in water aggregates from dimer to pentamer are seen in our spectra. Following an increase in argon backing pressure, the position of the cluster absorption bands varies from those characteristics of isolated water aggregates in the gas phase to those known for clusters trapped in a static argon matrix. These variations can be interpreted in terms of sequential solvation of the water clusters by an increasing number of argon atoms attached to water clusters. Our measured spectra are in good agreement with those obtained previously either for free or Ar coated small-sized water clusters using pulsed slit-jet expansions. Our results are equally in accord with those originating from a variety of tunable laser based techniques using molecular beams or free jets or from the study of water aggregates embedded in rare gas matrices. Distinctions are reported, however, and discussed. Ab initio calculations have made it possible to speculate on the average size of an argon solvation shell around individual clusters as well as on the development of the OH stretch vibrational shifts in mixed (H(2)O)(m)Ar(n) clusters having different compositions and architectures.

High-resolution cavity ringdown spectroscopy of the jet-cooled ethyl peroxy radical C2H5O2

We have recorded high resolution, partially rotationally resolved, jet-cooled cavity ringdown spectra of the origin band of the A-X electronic transition of both the G and T conformers of the perproteo and perdeutero isotopologues of the ethyl peroxy radical, C(2)H(5)O(2). This transition, located in the near infrared, was studied using a narrow band laser source (< or approximately 250 MHz) and a supersonic slit-jet expansion coupled with an electric discharge allowing us to obtain rotational temperatures of about 15 K. All four spectra have been successfully simulated using an evolutionary algorithm approach with a Hamiltonian including rotational and spin-rotational terms. Excellent agreement with the experimental spectra was obtained by fitting seven molecular parameters in each ground and the first excited electronic states as well as the band origin of the electronic transition. This analysis unambiguously confirms the assignment of the lower frequency origin band to the G conformer and the higher frequency one to the T conformer.

Infrared spectra of two isomers of OCS–C 2H 2 and OCS–C 2D 2 in the region of OCS ν1 fundamental

Infrared spectra of OCS–C 2H 2 and OCS–C 2D 2 complexes in the region of the C–O stretching fundamental of OCS (∼2060 cm −1) are studied in a pulsed supersonic slit-jet expansion using a tunable diode laser. For each complex, two bands are observed and assigned to distinct near-parallel and the T-shaped isomers. Ground state parameters were previously determined from microwave studies, so analysis of the infrared spectra gives information on the vibrational shifts upon complex formation as well as rotational and centrifugal distortion parameters for the excited states. All four bands show a red shift with respect to the monomer band origin, with the T-shaped isomer having a much larger shift than the near-parallel isomer. Disappearance of the T-shaped isomer when argon is used as a carrier gas supports the notion that the near-parallel isomer is the lowest energy form of the complex.

Selective Detection of Radicals and Ions in a Slit-Jet Discharge by Degenerate and Two-Color Four-Wave Mixing

Degenerate four-wave mixing (DFWM) was used to record the spectra of charged and neutral carbon-containing radicals generated in a pulsed discharge source within a supersonic slit-jet expansion. Detection limits of approximately 10(9) molecules cm(-3) are achieved. The DFWM method allows a selective molecular detection by varying the discharge timings. Increased spectral selectivity is obtained by applying the two-color, doubly resonant four-wave mixing variant. This shows the potential of the techniques for sensitive and selective spectral analysis of radicals in discharges. The methods are successfully used for the detection of C(4)H, HC(2)S, and HC(4)H(+) with signal-to-noise in the range of 10(2)-10(4).

Infrared spectra of the OCS-CO2 complex: Observation of two distinct slipped near-parallel isomers

Infrared spectra of OCS-CO(2) complexes are studied in a pulsed supersonic slit-jet expansion using a tunable diode laser probe in the 2060 cm(-1) region of the C-O stretching fundamental of OCS. Two bands are observed and analyzed, corresponding to two distinct isomers of the complex. Isomer a is the known form which has been previously studied in the microwave region. Isomer b is a new form, expected theoretically but first observed here. Structures are determined with the help of isotopic substitution. Both isomers are planar, with slipped near-parallel geometries. In isomer a, the intermolecular (center of mass) separation is 3.55 A and the C atom of the CO(2) is closer to the S atom of the OCS. In isomer b, the C atom of CO(2) slides closer to the O atom of OCS and the center of mass separation increases to 3.99 A. Isomer a is the lowest energy form, but paradoxically isomer b appears to be stronger in our infrared spectra. Predicted pure rotational transition frequencies are given to help in a search for the microwave spectrum of isomer b.

Conformation and Aggregation of Proline Esters and Their Aromatic Homologs: Pyramidal vs. Planar RR´N-H in Hydrogen Bonds

Abstract The conformations of proline esters are investigated by infrared spectroscopy in supersonic slit jet expansions. Two easily convertible puckering variants of the pyrrolidine ring with intramolecular N-H···O contacts are shown to be particularly stable. The aggregation tendency of proline esters via intermolecular N-H···O hydrogen bonds is remarkably weak. IR differences between enantiopure and racemic dimers are difficult to quantify. Dehydrogenation of the pyrrolidine ring to pyrrole leads to a stable planar carboxylic ester conformation. Its aggregation tendency is pronounced due to the planar hybridization of the nitrogen atom and leads to a symmetric, β sheet-like dimer with strongly red-shifting hydrogen bonds. The spectroscopic observations underscore the differences between intermolecular interactions of N-terminal and peptide-bound amino acids in peptide chains.

The weakly-bound nitrous oxide–acetylene complex: Fundamental and torsional combination bands of N 2O–C 2H 2 and N 2O–C 2D 2 in the N 2O ν1 region

Spectra of the weakly-bound N 2O–C 2H 2 and N 2O–C 2D 2 complexes in the region of the N 2O ν 1 fundamental band (∼2224 cm −1) are observed in a pulsed supersonic slit jet expansion probed with a tunable diode laser. Two bands are analyzed for each complex: the fundamental (N–N stretch), and a combination involving the intermolecular torsional (out-of-plane bend) vibration. The resulting torsional frequencies are 44.37 and 40.01 cm −1 for the C 2H 2 and C 2D 2 complexes, respectively. This represents the first observation of the N 2O–C 2D 2 isotopomer, and the first direct determination of an intermolecular frequency for nitrous oxide–acetylene.

New infrared spectra of the nitrous oxide trimer

Infrared spectra of N(2)O trimers are studied using a tunable diode laser to probe a pulsed supersonic slit-jet expansion. A previous observation by Miller and Pedersen [J. Chem. Phys. 108, 436 (1998)] in the N(2)O nu(1)+nu(3) combination band region ( approximately 3480 cm(-1)) showed the trimer structure to be noncyclic, with three inequivalent N(2)O monomer units which could be thought of as an N(2)O dimer (slipped antiparallel configuration) plus a third monomer unit lying above the dimer plane. The present observations cover the N(2)O fundamental band regions nu(3) ( approximately 1280 cm(-1)) and nu(1) ( approximately 2230 cm(-1)). In the nu(3) region, two trimer bands are assigned with vibrational shifts and other characteristics similar to those in the nu(1)+nu(3) region, but in the nu(1) region all three possible trimer bands are observed. Relationships among the various bands are considered with reference to their rotational intensity patterns, their vibrational shifts, and the properties of the related N(2)O dimer, with results that generally support the conclusions of Miller and Pedersen. Three trimer bands are also observed for the fully (15)N-substituted species in the nu(1) region, and these results should aid in the detection of the as-yet-unobserved pure rotational microwave spectrum of the trimer.

Nonpolar nitrous oxide dimer: fundamentals of the mixed 14N2O–15N2O dimer and new combination bands of (14N2O)2 and (15N2O)2 involving the Bu intermolecular bend

Spectra of the nonpolar nitrous oxide dimer in the region of the N2O v1 fundamental band are observed in a pulsed supersonic slit jet expansion probed with a tunable diode laser. Four bands are analysed: two fundamentals of the mixed 14N2O-15N2O dimer and combination bands involving the intermolecular disrotation of the monomers (Bu intermolecular bend) for both (14N2O)2 and (15N2O)2. Because the determination of this intermolecular frequency relies on the experimentally unknown frequency of the (forbidden) symmetric fundamental, we used previously published ab initio results and their proximity to our experimental values to assign the upper state of the combination bands. The resulting intermolecular disrotation frequencies are 42.3(1.0) and 41.6(1.0) cm(-1) for the (14N2O)2 and (15N2O)2, respectively. This represents the first observation of the mixed 14N2O-15N2O dimer, and the direct determination of a second intermolecular frequency for the nonpolar (N2O)2.

The ˜A 2Π3/2 − ˜X 2Π3/2 electronic transition of HC4S isotopologues

The electronic transition of several isotopologues of the HC4S radical (DC4S, HC4 34S, DC4 34S) produced in a pulsed supersonic slit-jet expansion discharge has been measured by cavity ringdown spectroscopy. The sensitivity of the technique and a higher rotational temperature enables observation of weaker features in the spectrum of HC4S not previously detected by laser induced fluorescence. Rotational fits were made on several of these hot bands as well as on the origin bands of the four isotopologues. The high signal-to-noise ratio of the spectra enables precise determination of line positions. Band origin and effective rotational constants were found to be T 00 = 19981.441(1), B'eff = 0.04540(1), B''eff = 0.04661(1) cm−1 for HC4 34S, and T 00 = 20035.630(4), B'eff = 0.04318(5), B''eff = 0.04432(4) cm−1 for DC4 34S. The quartic centrifugal distortion term in the v = 0 of the excited state is 1.50(36) × 10−9 for HC4S and 1.58(43) × 10−9 cm−1 for DC4S. Measurements made at different temperatures combined with theoretical calculations suggest that the spin orbit splitting in the ground state is likely to be close to −270 cm−1 rather than the much smaller value inferred indirectly earlier.

Weak Hydrogen Bonds Make a Difference: Dimers of Jet-Cooled Halogenated Ethanols

Abstract Hydrogen-bonded clusters of fluorinated and chlorinated ethanols exhibit rich isomerism in terms of monomer conformation, secondary contacts between the OH and CH groups and the halogen atoms, hydrogen bond topology, chirality recognition and acceptor lone electron pair choice. By expanding the six alcohols involving one to three fluorine or chlorine atoms at the methyl group in a supersonic slit jet expansion and by probing their monomer, dimer and trimer IR spectra between 800 and 4000 cm−1, this isomerism is unravelled in substantial detail. Argon relaxation experiments and complementary cluster Raman spectroscopy provide further information on the individual dimer conformations and on trimer assignments. Energy sequences, helicity- and topology-dependent OH red-shifts, differences between fluorine and chlorine, the influence of dispersion-like interactions and halogen number trends are uncovered and compared to systematic quantum-chemical calculations up to MP2/6–311+G* level. The experimental data provide rigorous reference values for an accurate and balanced quantum-mechanical description of weak hydrogen bond interactions to halogens in the presence of a strong hydrogen bond between oxygen atoms.

High-resolution infrared spectroscopy of jet-cooled vinyl radical: Symmetric CH2 stretch excitation and tunneling dynamics

First high-resolution IR spectra of jet-cooled vinyl radical in the C-H stretch region are reported. Detailed spectral assignments and least squares fits to an A-reduction Watson asymmetric top Hamiltonian yield rotational constants and vibrational origins for three A-type bands, assigned to single quantum excitation of the symmetric CH(2) stretch. Two of the observed bands arise definitively from ground state vinyl radical, as rigorously confirmed by combination differences predicted from previous midinfrared CH(2) wagging studies of Kanamori et al. [J. Chem. Phys. 92, 197 (1990)] as well as millimeter wave rotation-tunneling studies of Tanaka et al. [J. Chem. Phys. 120, 3604 (2004)]. The two bands reflect transitions out of symmetric (0(+)) and antisymmetric (0(-)) tunneling levels of vinyl radical populated at 14 K slit-jet expansion temperatures. The band origins for the lower-lower (0(+)<--0(+)) and upper-upper (0(-)<--0(-)) transitions occur at 2901.8603(7) and 2901.9319(4) cm(-1), respectively, which indicates an increase in the tunneling splitting and therefore a decrease in the effective tunneling barrier upon CH(2) symmetric stretch excitation. The third A-type band with origin at 2897.2264(3) cm(-1) exhibits rotational constants quite close to (but at high-resolution distinguishable from) the vinyl radical ground state, consistent with a CH(2) symmetric stretch hot band built on one or more quanta of excitation in a low frequency vibration. The observed CH(2) symmetric stretch bands are in excellent agreement with anharmonically scaled high level density functional theory (DFT) calculations and redshifted considerably from previous low resolution assignments. Of particular dynamical interest, Boltzmann analysis indicates that the pair of 0(+) and 0(-) tunneling bands exhibits 1:1 nuclear spin statistics for K(a)=even:odd states. This differs from the expected 3:1 ratio for feasible exchange of the two methylenic H atoms but is consistent with a 4:4 ratio predicted for interchange between all three H atoms. This suggests the novel dynamical possibility of large amplitude "roaming" of all three H atoms in vinyl radical, promoted by high internal vibrational excitation arising from dissociative electron attachment in the discharge.

Electronic spectra of radicals in a supersonic slit-jet discharge by degenerate and two-color four-wave mixing

Four-wave mixing techniques have been used for the measurement of electronic transitions of cold transient species generated in a supersonic slit-jet discharge expansion. The origin band of the d(3)Pi(g)-a(3)Pi(u) system of C(2) and A(2)Pi(3/2)-X[combining tilde](2)Pi(3/2) electronic transition of HC(4)S were recorded. A signal-to-noise ratio of 10(4) in the spectra was achieved, resulting in detection limits of 10(10) cm(-3) for these two molecules. Application of selective two-color resonant four-wave mixing is used for the spectral assignment utilizing the double-resonance nature of the method. The combination of these techniques with a slit source proves to be a sensitive approach for the detection of transient molecules in a molecular beam discharge.

Rotationally resolved infrared spectroscopy of a jet-cooled phenyl radical in the gas phase

The first high-resolution IR spectra of a jet-cooled phenyl radical are reported, obtained via direct absorption laser spectroscopy in a slit-jet discharge supersonic expansion. The observed A-type band arises from fundamental excitation of the out-of-phase symmetric CH stretch mode (nu19) of b2 symmetry. Unambiguous spectral assignment of the rotational structure to the phenyl radical is facilitated by comparison with precision 2-line combination differences from Fourier transform microwave and direct absorption mm-wave measurements on the ground state [R. J. McMahon et al., Astrophys. J., 2003, 590, L61]. Least-squares fits to an asymmetric top Hamiltonian permit the upper-state rotational constants to be obtained. The corresponding gas-phase vibrational band origin at 3071.8904 (10) cm(-1) is in remarkably good agreement with previous matrix isolation studies [A. V. Friderichsen et al., J. Am. Chem. Soc., 2001, 123, 1977], and indicates only a relatively minor red shift (approximately 0.9 cm(-1)) between the gas and Ar matrix phase environment. Such studies offer considerable promise for further high resolution IR study of other aromatic radical species of particular relevance to combustion phenomena and interstellar chemistry.