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J . Phys. Chem. 1994,98, 4800-4808
Vibrational Spectroscopy of C-H Bonds of C2H4 Liquid and C2H4 in Liquid Argon Solutions Ansgar Brock, Nairmen Mina-Camilde, and Carlos Manzanares I' Department of Chemistry, Baylor University, Waco. Texas 76798 Received: December 14, 1993; In Final Form: March 2, 1994'
The spectra of the fundamental and first and second overtones (Av = 1, 2, and 3) around the C-H stretch of ethylene in liquid argon solutions have been measured a t 88 K. The IR, near-IR, and visible spectra of pure liquid ethylene have been measured in the range from 2800 to 15 000 cm-I a t 109 K. Absorptions in the visible were obtained with a low-temperaturecell and a resonant continuous wave laser technique with acoustic detection. Absorptions below 12 000 cm-I were observed at low temperatures with a Fourier transform spectrophotometer operating in the near-IR region. Comparison is made between the absorption bands of gas phase, liquid argon solution, and liquid phase ethylene. Spectra of solutions of ethylene in liquid argon show great simplification of the bands with respect to the room temperature gas-phase absorption bands. Due to the small frequency shift in solution, assignments are presented based on gas-phase measurements. For the C-H transitions of pure liquid ethylene, the local mode harmonic frequency and anharmonicity are obtained and used with a harmonically coupled anharmonic oscillator (HCAO) model to calculate the energy of the levels and assign the absorption bands to particular transitions.
Introduction The gas-phase infrared, near-infrared, and visible spectra of C-H and C-D stretch vibrations of ethylene and isotopic ethylenes have been the subject of several investigations.1-l4 The spectra of the molecules C2H3D and CzHD3 have been interpreted below 9500 cm-1 in terms of a local mode model which involves different Fermi resonance interactions in each molecule.' Duncan and Robertson2 have made normal mode assignments for ethylene below 6000 cm-I. Duncan and Ferguson3 studied the C H and CD stretching vibrations up to Av = 6 in CzH4 and Av = 4 in CzD4. The interpretation was done in terms of local mode parameters taking into account major Fermi resonances. The C H and CD vibrational frequencies of HzCCD2 have been studied4 up to Av = 6 (CH) and Av = 3 (CD), interpreting the absorptions in terms of a transition from normal to local modes. Jasinski5 obtained the fifth overtone spectra of ethylene and deuterated ethylenes. The Raman spectra of CzH4 have been measured6 between 800 and 6300 cm-I. The I R spectra of C2HsD have been recorded7 at high resolution around 3000 cm-I. An anharmonic force field has been proposed8 to obtain the experimental frequenciesof ethylene anddeuteratedethylenes. High-resolution IR spectra of the u7 and Y I O perpendicular fundamental bands were reported by Smith and Mills.9 A rotational resolution I R study was done by Allen and Plyler.Io The Raman spectrum was reported by Welsh, Romanko, and Feldman" under high resolution. Assignments of some combination bands were also given by Plyler.Iz Thevibrational assignments were reinvestigated by Arnett and Crawford." The I R and Raman spectra of ethylene before 1945 have been summarized by Herzberg.14 In condensed phases, the visible spectrum of liquid ethylene was reported by Nelson and Pate115 between 14 000 and 17 000 cm-l with some tentative assignments. The IR spectra of crystalline CzH4 and C2D4 was studied by DowsI6 a t 65 K. The Raman spectrum of the liquid was obtained in the fundamental region by Rank, Shull, and Axford.17 The experimental and calculated results mentioned before have allowed the determination of the energy levels and states of the C-H fundamental and overtone spectra of gas-phase ethylene in terms of the local mode description. It is easy to notice that very little information about ethylene has been obtained in condensed phases at low temperatures. Low-temperature studies have added *Abstract published in Advance ACS Abstracts, April 1, 1994.
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importance because of the fact that if the interactions in the liquid are relatively weak, there will be a decrease of band overlapping. Bulanin18 has shown, by comparison between the IR spectra of pure samples a t low temperature and dilute solutions of the same samples in liquefied gases, that the dilute solution provides a good approximation to the pseudo gas phase. Interactions between molecules of the same kind disappear. Absorption bands are very sharp, and there is a decrease in band overlapping. The rotational band envelope collapses because of the low temperatures and the hindering of the rotation in the presence of the solvent. Under these conditions, it is often possible to obtain more accurate vibrational terms than from the gas-phase spectra with unresolved rotational structure. The work of Bulanin has been limited to mainly fundamental bands below 4000 cm-1. There are a few reports of high-energy overtones studied in liquid solutions at low temperatures. With our studies we expect to compare the fundamental and overtone spectra of CzH4 in gas, in liquid argon solution, and in pure liquid forms to identify some hidden bands that are present in the room temperature studies but cannot be seen due to rotational congestion. If the relative interactions in condensed phases are weak, the gas-phase description could still be used with a better knowledge of the vibrational band origins. The study of samples in solutions of liquefied inert gases could also, in principle, give information about the extent of inhomogeneous contribution to absorption bands of the room temperature gas-phase spectrum. With these considerations in mind, the C-H fundamental and overtone spectra of ethylene were obtained in liquid argon solution and in pure liquid form. The C-H vibrational overtone transitions between 2800 and 10 000 cm-1 of ethylene are obtained in diluted argon solutions at 88 K and compared with gas-phase spectra obtained at room temperature. The C-H overtone transitions between 2800 and 15 000 cm-* are obtained for pure liquid ethylene at 109 K and are also compared to the gas-phase spectra. It is shown that, in very dilute solutions at low temperatures, the absorption bands are easier to interpret than the room temperature gas-phase bands, because of the reduction in the contribution of the rotational envelope and the hot bands. The simplification of the spectra allows the observation and assignment of hidden transitions in the room temperature absorptions. The experimental results obtained for pure liquid ethylene could also be used in connection with studies of the atmospheres of the outer planets. The encounters of Voyager 1 and 2 with the Saturn 0 1994 American Chemical Society
Vibrational Spectroscopy of C-H Bonds of C2H4 and Jupiter planetary systems and Voyager 2 with the Neptune and Uranus planetary systems confirmed the presence of hydrocarbons in the atmospheres of those planets and some of their satellites.l9-27 The temperatures of these planetary systems are such that methane, ethane, ethylene, acetylene, acetylene, and other hydrocarbons could exist as liquid droplets in the atmosphere and as solid or liquids on the surface. In the laboratory, only a few determinations have been made of the near-infrared and visible absorption of liquefied and solidified gas-phase hydrocarbons. Due to the low temperatures involved, the spectroscopic measurements of liquid ethylene that are presented in this workcould also be used to interpret thevibrational spectra of hydrocarbons found in the major planets of the solar systems.
Experimental Section The ethylene samples were studied using two different techniques. Fundamental and overtone absorptions below 10 000 cm-l of the samples in gas phase, liquid argon solution, and liquid phase were observed with a Fourier transform spectrophotometer operating in the infrared and near-infrared regions. Absorptions above 10 000 cm-I were obtained with a laser optoacoustic spectrometer. The description of the techniques and lowtemperature cells employed as well as details of the preparation of the solutions will be given separately. A. FT Spectrometer. Spectra of the C-H stretching in the near-IR region were obtained using a Mattson Fourier transform spectrophotometer in the range from 4000 to 12 000 cm-I. The spectrophotometer operates with a PbSe detector, a tungsten light source, and a quartz beam splitter. The gas-phase C-H spectra around Au = 1,2,3, and 4 were recorded at a resolution of 2 cm-l, using a variable path length cell. The spectra in liquid argon solutions were obtained with a 10-cm-path length cell with sapphire windows. The pure liquid phase spectra around AD = 2 and 3 were obtained with a 1.8-cm-path length cell and around Au = 4 with a 10-cm-path length cell. B. Acoustic Spectrometer. The output beam of a Laser Ionics 544A C W argon ion laser operating in the all lines mode is used to pump a CW Coherent 599-01 dye laser. The power of the pump beam is 5 W, and the power of the dye laser is approximately 100 mW. An acoustooptic modulator (AOM) system (Newport modulator and driver) is used to modulate the dye laser beam from 10 to 200 kHz as a square wave. Wavelength tuning of the (1-cm-1 bandwidth) dye laser is accomplished with a birefringent filter driven by a stepper motor. The stepper motor is controlled with a microcomputer. The modulated output of the dye laser is directed along the length of a photoacoustic cell. The piezoelectric transducer is coupled into a high impedance preamplifier with voltage gain 14. The diagram for the preamplifier is similar to the one given by Winefordner et a1.28 The signal from the piezoelectric and preamplifier system is amplified (X20) with a Stanford Research amplifier (SR560) and fed to an Ithaco (3962A) lock-in amplifier with 200-kHz frequency response. The modulated dye laser beam passes through the cell and is detected with a fast silicon photodiode and processed with another Ithaco (3962A) lock-in amplifier. The reference signal to both lock-in amplifiers is obtained from a square wave signal generator (SG) which also supplies the modulation signal to the driver input. Normalization of the photoacoustic signal is achieved by dividing the output voltages (A/B) of both lock-in amplifiers. A microcomputer system controls the dye laser wavelength scan and digitizes and stores the normalized signal as a function of the wavelength for further analysis. Pyridine 2 was used as the laser dye in the range 13 O W 14 500 cm-1. Absolute calibration of the dye laser lines was achieved by comparison with the optogalvanic spectrum of a hollow cathode lamp filled with neon.29 Ethylene (99.97% purity) and argon (99.99% purity) were obtained from Matheson.
The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4801
C. Low-Temperature Cells. An APD Cryogenics Heli-tran Model LT-3-110 system was used for sample cooling. The cylindrical photoacoustic cell is 12.7 mm in diameter and 16 mm in length. The cell is bored out of a copper cube. The transducer, similar to the one described by Tam and Patel,30is inserted radially at the center of one side of the cube. Only a polished face of the transducer is in contact with the liquid sample. The cell has sapphire end windows (20-mm diameter) sealed with indium O-rings and fastened to the cell with flat square flanges. The piezoelectric material is a lead zirconate-titanate disc of 3-mm thickness and 10-mm diameter purchased from Transducer Products. The cell was fastened to the cryostat cold head. Thermal isolation was achieved by having the cell in an evacuated (
