J. Phys. Chem. 1981, 85,761-762
traction in the vapor phase due to the presence of HzO causes the critical point to rise. In CzH6 + HzO, the three-phase line lies at a temperature lower than Tc for pure CzH6 (see Figure 2). The phase diagrams for short-chained hydrocarbons (C, to C8, at least) fall into this class. The topology of such a diagram does not preclude either a rise or fall in F upon the addition of water; however, other alkane + H 2 0 mixtures appear to behave much like ethane. The temperature at the end of the critical locus reported for n-C4Hio + HzO by Reamer et al. is the same as the critical temperature of pure n-C4Hlo, within the sensitivity of their experiment.1° Water at these very small concentrations affects
76 1
the alkanes not as a highly polar hydrogen-bonding material, but as a small relatively unpolarizable molecule; indeed, its effect is the same that Nz would be expected to have on the critical point. For example, low concentrations of Nz in n-butane raise p c but leave Tc effectively unchanged.ll Acknowledgment. This work was supported in part by the Department of Energy (DOE Contract No. P7901196). (10) H. H. Reamer, R. H. Olds, B. H. Sage, and W. N. Lacey, Ind. Eng. Chem., 36, 381 (1944). (11) W. W. Akers, L. L. Attwell, and J. J. McKetta, Ind. Eng. Chern., 46, 2539 (1954).
Selective Multiphoton Ionization of Geometric Isomers: cis- and trans-I ,2-Dichloroethene Jeffrey W. Hudgens,” Mark Seaver,+ and J. J. DeCorpo Laser Chemistry Group, Code 61 10, Naval Research Laboratoty, Washington, D.C. 20375 (Received: February 2, 1981)
Mass spectral resolution of the cis and trans isomers of 1,2-dichloroethene is demonstrated by the use of multiphoton ionization (MPI). These isomers are resolved by using three photon resonances predicted from vacuum UV spectroscopy of each isomer. A fourth photon is absorbed to form the ions. A plot of isomer selectivity vs. wavelength shows that discrimination of one isomer from the other of >lo1 can be realized. The MPI mass spectra of ions formed from both isomers reveal identical fragmentation patterns with wavelength (440-465 nm). The MPI mass spectrum shows greater elimination of C1 than observed under electron impact ionization.
Introduction Several authors, including ourselves, have suggested that geometric isomers which produce identical mass spectra with electron impact ionization can be identified by their electronic absorption fingerprint using multiphoton ionization (MPI) in a mass ~pectrometer.l-~One study along these lines observed the CloHs isomers azulene and naphthalene and found the fragmentation patterns dependent on ~ a v e l e n g t h .However, ~ these two CloHs isomers are distinguishable under conventional electron impact ioni~ation.~?~ Currently, mass spectral identification of geometric isomers in mixtures requires prior separation by gas chromatography. Reference tabulations5 of the fragmentation patterns of cis- and trans-1,2-dichloroethene, as well as measurements in our mass spectrometer (Table I), show the isomer pair of this study to have identical fragmentation patterns. Experimental Section The principal characteristics of the experimental apparatus are described e1sewhere.l Briefly, it consists of a nozzle beam, a Nd:YAG pumped dye laser beam focused with a 50-mm lens, and a quadrupole mass spectrometer. The molecular beam and laser beam crossed at right angles within the ionizer. A diffusion pump and 77 K beam traps maintained the pressure of the quadrupole region of the mass spectrometer at less than 5 x torr. A filament below the beam plane provided electron impact data. Gas chromatography showed the samples of cis- and trans1,2-dichloroethene to contain about 5% isomer cross NR,L-NRC Postdoctoral Associate, 1978-1980. Sterling Chemistry Laboratory, Yale University, New Haven, CT 06511.
contamination, -10% CHC13, and 5% CC4.
Results and Discussion The total ion signal spectra of trans and cis isomers between 440-465 nm at 0.5-cm-l resolution are shown in Figure 1. In both molecules the MPI spectrum arises from a three-photon resonance with a Rydberg state followed by absorption of a fourth photon into the ionization continuum. These resonance states were studied previously by Walsh and Warsop using vacuum UV spectroscopy and were assigned as either the (a,4s) or (a,3d) transition^.^^' These one-photon assignments are indicated on the spectra. The strong transition in cis-1,2-dichloroethene which Walsh labeled “A”, but could not assign, appears in these results at 460.2 nm. Considerable structure which cannot be assigned by comparison with the vacuum UV spectra is also seen. These features are currently under further investigation. Toward shorter wavelength the trans isomer shows a continuum of increasing intensity. Figure 2 shows the isomeric selectivity vs. wavelength. Selectivity is represented as the log of the ratio of trans to cis isomer total ion signals. Greater discrimination of (1) M. Seaver, J. W. Hudgens, and J. J. DeCorpo, Int. J.Mass Spectrom. Ion Phys., 34, 159 (1980). ( 2 ) J. H. Brophy and C . T. Rettner, Chem. Phys. Lett., 67,351 (1979). (3) U. Boesl, H. J. Neusser, and E. W. Schlag, J. Chem. Phys., 72,4327
(1980). (4) D. M. Lubman, R. Naaman, and R. N. Zare, J. Chem. Phys., 72, 3034 (1980). (5) A. Cornu and R. Massot, “Compilation of Mass Spectral Data”, Vol. I, 2nd ed, Heyden, London, 1975. (6) A. D. Walsh and P. A. Warsop, Trans. Faraday Soc., 63, 524 (1967). (7) A. D. Walsh and P. A. Warsop, Trans. Faraday SOC.,64, 1418 (1968).
This article not subject to US. Copyright. Published 1981 by the American Chemical Society
762
The Journal of Physical Chemistry, Vol. 85, No. 7, 1981
Letters
TABLE I: Mass Spectra of cis- and trans-1,2-DichloroetheneObtained by Electron Impact or Multiphoton Ionization mass/charge4
ionization method EIb EIb MPIC MPI~
molecule trans-l,2-dichloroethene cis-1,2-dichloroethene trans-1,2-dichloroethene cis-1,2-dichloroethene
61 43 43 tr tr
96 23 25 tr tr
98
60
26
25
35
24
14 13 tr tr
10 9 tr tr
8.5 8 65 65
1.5 1.5 19 19
tr tr
15 15
Relative abundance given as the percentage of the total ion signal. The mean electron energy was 25 eV. The average of four mass spectra taken with excitation at 455.7, 458.6, 451.8, and 451.0 nm. Relative error bars are roughly i25%. The average of four mass spectra taken with excitation at 449.7, 460.2, 455.3, and 445.8 nm. Relative error bars are roughly t 25%.
trans
5;
I
1""I""I""I""I""l 440 445 450 455
460
465
nm
Figure 2. The log ratio of trans to cis ion signal strengths vs. wavelength. Ratlos > 0 indicate enhanced selectivity for the trans isomer. Ratios < 0 indicate enhanced cis Isomer selectivlty.
440
445
450
455
460
465
nm
Figure 1. Total ion signal spectra of trans- (top trace) and c k - (bottom trace) 1,2-dichIoroethene vs. wavelength. Asslgnments are from ref 6 and 7.
one isomer from the other is observed at longer wavelengths where the trans isomer continuum is at a minimum. Clearly selectivities of >10:1 are easily obtained. The optimum selectivity for the cis isomer occurs at 460.2 nm ("A") and 449.9 nm (4;). Maximum selectivity for the
trans isomer is at 458.5 nm (0,O)and 455.7 nm (5;). A t each of these wavelengths a strong feature in one isomer corresponds to a weak or nonexistent absorption in the other isomer. Selectivity might be enhanced by narrower band excitation. Sensitivity could be improved by cooling the molecules with a seeded nozzle beam. The MPI fragmentation patterns for both isomers are nearly identical and appear invariant with wavelength (within S/N) over the limited spectral range reported here. Table I shows this data for both electron impact and MPI results. Most remarkable is that MPI prepared ions have shed their C1 atoms either through sequential elimination or by four-center elimination. As a result, the CzHz+ion becomes the base peak of MPI mass spectra of 1,2-dichloroethenes. Further studies of fragment ion yield vs. intensity may resolve whether this fragmentation occurs upon formation of the ion, or at the intermediate resonant state of the neutral molecules.
