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Formation of XeCl− in the gas phase
CHEhlICAL PHYSICS LETTERS
Volume 20, number 4
FORMATION
OF XeCl-
IN THE GAS PHASE
Jose M. RIVEROS, Peter W. TIEDEMANN and A. Celso BREDA Institute de Qxitnica, University of Siio Parrlo, SZ?O P&o,
Brasil
Received 12 March 1973
Ion cyclotron resonance has been used to detect rhc formation of XeCl- at pressure lo4 torr from the ionmolecule reaction of COW and Xe. The dissociation energy of XeCI- into Xc and Cl- is estimated to be Iess th3.n 10 kcal/mole. The results suggest that other negative ions similar to X&I- may be detected by mass spectrometric techniques.
Noble gas compounds are by now well characterized chemical systems in terms of structural parameters and bonding properties [ 1, 21. The stability of positive ions containing rare gas atoms is also known from mass spectroscopy [3,4], and particularly by the extensive studies of ion-molecule reactions leading to the formation of protonated species, XH+ [5]. More recent investigations have been successful in studying ion-molecule reactions which yield XF+ [4] and XCH; [7] as stable products, and estimates have been made on the bond energies of these ions. On the other hand, little information is available on negative ions of noble gas compounds. In the present
communication,
we wish
to report
the observation in the gas phase of a negative ion, XeCl-, a type of species which hereto has been relatively unknown. The formation of such an ion has been accomplished in a conventional ion cyclotron resonance spectrometer by application of a general technique that has proved to be very convenient for the indirect association of chloride ions with neutrals [S] . The method is based on the reaction of COCl-, obtained from phosgene by impact of 30 eV electrons, with excess Xe at a total pressure of 1O-4 torr. COCl- t Xe + XeCl- + CO . The partial pressure of phosgene was maintained at 3 X, 10e6 torr, and the marginal oscillator detector was kept at 122 kHz. The ICR spectrum of the phosgenexenon mixture displays clearly a number of peaks in
the high mass range: r?z/e 164 (12gXe35C!-), m/e 166 (131Xe35C1- and !zgXe37C1-), nzje 167 (132Xe35Cl-) and m/l I69 (132Xe37CI- and 134Xe35CI-)_ The spectrum is partially unresolved under the experimental conditions because of the high pressure and the low operating frequency. The bond energy of XeCl- is of special interest because in a valence bond picture this ion corresponds to the ir;teraction of two completely closed shell atoms. Although this quantity c )uTd be obtained in principle by the use of ion trapping techniques in ICR [9], the weakness of the signa and th=iinstability of the Xe pressure due to our ion pumping, make such a measurement meaningless. By comparison with Cl-(CH,CN) for which an accurate value is available for the ion-neutral bond energy [IO], and which has been observed in our previous study [8], the bond energy of XeCl- can be estimated to be less than 10 kcal/mole (XeCl- + Xe+CI-). It is interesting to notice that a reasonable value for the bond energy in XeCl- with respect to the experimenta! upper limit, is predicted by a simple model of a spherical charge (Cl-) interacting witi an isotropic polarizable atom (Xe). Thus, a calculation including the ion-induced dipole attraction ofCI_ and Xe, the corresponding London dispersion energy as modified by Pitzer [ll, 121, and the repulsive potential obtained from Cl--Xe scattering experiments [ 131 yields an energy minimum at 2.85 A and a bond energy of 7.5 kcal/moIe. Since this tiodel essentially 345
Volume 20. number 4
CHEMICAL PHYSICS LETTERS
neglects the effect of exchange integrals, molecular orbital calculations are presently under way to obtain a more realistic picture of the bonding in XeCl-. Our results suggest that other negative ions of the type (X-halide)- may ‘oe detected in the gas phase. Similar calculations to those performed for XeCIpredict a bond energy of the order of 3 kcal/mole for KrCI- , The detection of this species, as well as XeFand KrF-, should presumably be more favourable by direct clustering reactions in a high-pressure m2s.s spectrometer [ 101.
We would like to thank Conselho National de Pesquisas for support of this research, and the Funda$Bo de Amparo 1 Pesquisa do Estado de S2o Pauio for a graduate fellowship (ACB).
lSJune1973
References [l] J.H. Holloway, Progr. Inorg. Chem. 6 (1964) 241. [2] N. Bartlett, Endeavour (Spanish Edition) 31 (1972) 107. i3] CL. Chernick, H.H. Claasen,P.R. Ftelds, H.H. Hyman, J.G. Maim, W.M. Manning, MS. Matheson, L.A. Quarterman, F. Schreiner, H.H. Wig, I. Sheft, S. Siegel, E.N. Sloth, L. Stein, M.H. Studier, J.L. Weeks and M.H. Zirin, Science 138 (1962) 136. 141 h1.H. Studier and E.N. Sloth, J. Phys. Chem. 67 (1963) 325. E.W. McDaniel, V. Cermak, A. Dalgarno, E.E. Ferguson lnd L. Friedman, Ion-molecule reactions (Wiley-Interscience, New York, 1970) ch. 5. 161 I. Berkowitz and W.A. Chupka, Chcm. Phys. Letters 4 (1970) 447. [71 D. Holtz and J.L. Beauchamp, Science 173 (1971) 1237. [al J.M. Riveros, AC. Bredn and LX. Blair, submitted for publication. 191 R.T. McIver Jr. and J.R. Eyler, J. Am. Chem. Sot. 93 (1971) 6334. 1101 R. Yamdagni and P. Kebarle, J. Am. Chem. Sot. 94 (1972) 2940. [Ill KS. Pitzer, Advan. Chem. Phys. 2 (1959) 59. [I21 I. Eliezer and P. Krindel, J. Chem. Phys. 57 (1972) 1884. I131 KG. Spears, J. Chem. Phys. 57 (1972) 1842, 1850.
Volume 20, number 4
FORMATION
OF XeCl-
IN THE GAS PHASE
Jose M. RIVEROS, Peter W. TIEDEMANN and A. Celso BREDA Institute de Qxitnica, University of Siio Parrlo, SZ?O P&o,
Brasil
Received 12 March 1973
Ion cyclotron resonance has been used to detect rhc formation of XeCl- at pressure lo4 torr from the ionmolecule reaction of COW and Xe. The dissociation energy of XeCI- into Xc and Cl- is estimated to be Iess th3.n 10 kcal/mole. The results suggest that other negative ions similar to X&I- may be detected by mass spectrometric techniques.
Noble gas compounds are by now well characterized chemical systems in terms of structural parameters and bonding properties [ 1, 21. The stability of positive ions containing rare gas atoms is also known from mass spectroscopy [3,4], and particularly by the extensive studies of ion-molecule reactions leading to the formation of protonated species, XH+ [5]. More recent investigations have been successful in studying ion-molecule reactions which yield XF+ [4] and XCH; [7] as stable products, and estimates have been made on the bond energies of these ions. On the other hand, little information is available on negative ions of noble gas compounds. In the present
communication,
we wish
to report
the observation in the gas phase of a negative ion, XeCl-, a type of species which hereto has been relatively unknown. The formation of such an ion has been accomplished in a conventional ion cyclotron resonance spectrometer by application of a general technique that has proved to be very convenient for the indirect association of chloride ions with neutrals [S] . The method is based on the reaction of COCl-, obtained from phosgene by impact of 30 eV electrons, with excess Xe at a total pressure of 1O-4 torr. COCl- t Xe + XeCl- + CO . The partial pressure of phosgene was maintained at 3 X, 10e6 torr, and the marginal oscillator detector was kept at 122 kHz. The ICR spectrum of the phosgenexenon mixture displays clearly a number of peaks in
the high mass range: r?z/e 164 (12gXe35C!-), m/e 166 (131Xe35C1- and !zgXe37C1-), nzje 167 (132Xe35Cl-) and m/l I69 (132Xe37CI- and 134Xe35CI-)_ The spectrum is partially unresolved under the experimental conditions because of the high pressure and the low operating frequency. The bond energy of XeCl- is of special interest because in a valence bond picture this ion corresponds to the ir;teraction of two completely closed shell atoms. Although this quantity c )uTd be obtained in principle by the use of ion trapping techniques in ICR [9], the weakness of the signa and th=iinstability of the Xe pressure due to our ion pumping, make such a measurement meaningless. By comparison with Cl-(CH,CN) for which an accurate value is available for the ion-neutral bond energy [IO], and which has been observed in our previous study [8], the bond energy of XeCl- can be estimated to be less than 10 kcal/mole (XeCl- + Xe+CI-). It is interesting to notice that a reasonable value for the bond energy in XeCl- with respect to the experimenta! upper limit, is predicted by a simple model of a spherical charge (Cl-) interacting witi an isotropic polarizable atom (Xe). Thus, a calculation including the ion-induced dipole attraction ofCI_ and Xe, the corresponding London dispersion energy as modified by Pitzer [ll, 121, and the repulsive potential obtained from Cl--Xe scattering experiments [ 131 yields an energy minimum at 2.85 A and a bond energy of 7.5 kcal/moIe. Since this tiodel essentially 345
Volume 20. number 4
CHEMICAL PHYSICS LETTERS
neglects the effect of exchange integrals, molecular orbital calculations are presently under way to obtain a more realistic picture of the bonding in XeCl-. Our results suggest that other negative ions of the type (X-halide)- may ‘oe detected in the gas phase. Similar calculations to those performed for XeCIpredict a bond energy of the order of 3 kcal/mole for KrCI- , The detection of this species, as well as XeFand KrF-, should presumably be more favourable by direct clustering reactions in a high-pressure m2s.s spectrometer [ 101.
We would like to thank Conselho National de Pesquisas for support of this research, and the Funda$Bo de Amparo 1 Pesquisa do Estado de S2o Pauio for a graduate fellowship (ACB).
lSJune1973
References [l] J.H. Holloway, Progr. Inorg. Chem. 6 (1964) 241. [2] N. Bartlett, Endeavour (Spanish Edition) 31 (1972) 107. i3] CL. Chernick, H.H. Claasen,P.R. Ftelds, H.H. Hyman, J.G. Maim, W.M. Manning, MS. Matheson, L.A. Quarterman, F. Schreiner, H.H. Wig, I. Sheft, S. Siegel, E.N. Sloth, L. Stein, M.H. Studier, J.L. Weeks and M.H. Zirin, Science 138 (1962) 136. 141 h1.H. Studier and E.N. Sloth, J. Phys. Chem. 67 (1963) 325. E.W. McDaniel, V. Cermak, A. Dalgarno, E.E. Ferguson lnd L. Friedman, Ion-molecule reactions (Wiley-Interscience, New York, 1970) ch. 5. 161 I. Berkowitz and W.A. Chupka, Chcm. Phys. Letters 4 (1970) 447. [71 D. Holtz and J.L. Beauchamp, Science 173 (1971) 1237. [al J.M. Riveros, AC. Bredn and LX. Blair, submitted for publication. 191 R.T. McIver Jr. and J.R. Eyler, J. Am. Chem. Sot. 93 (1971) 6334. 1101 R. Yamdagni and P. Kebarle, J. Am. Chem. Sot. 94 (1972) 2940. [Ill KS. Pitzer, Advan. Chem. Phys. 2 (1959) 59. [I21 I. Eliezer and P. Krindel, J. Chem. Phys. 57 (1972) 1884. I131 KG. Spears, J. Chem. Phys. 57 (1972) 1842, 1850.