424
Inorganic Chemistry, VoZ. 11, No. 2, 1972
NOTES
-717
w 2
-719 4
t
-72'
an orbital which in a higher symmetry environment could be characterized as a a-antibonding orbital of the organic moiety. Acknowledgment.-The financial support of Esso Research Laboratories, Humble Oil and Refining Co., Baton Rouge, La., is gratefully acknowledged. CONTRIBUTION FROM THE DEPARTMENT OF CHEMISTRY' CORNELL UNIVERSITY, ITHACA, NEW YORK 14850
T h e P r o t o n Affinity of Borazine ,
1
1
1
l
1
1
1
1
1
1
1
I
1
1
1
1
,
BY L. D. BETOWSKI, J. J , SOLOMON, AND RICHARD F. PORTER*
Received April 8, 1971 -94%
Chemical ionization mass spectrometry is a relatively new technique that has been used to provide information on the proton affinities of molecules. To date this technique has been used almost exclusively for studies of organic molecules, but i t should also be adaptable to inorganic systems. Borazine, an inorganic analog of benzene, has been the subject of many -100 theoretical and experimental investigations. We re0 cently reported results of a chemical ionization study of borazine that indicates that the proton affinity of this molecule was in excess of 7.7 eV.l More quantitative measurements of the proton affinity of borazine may be relevant to both its electronic structure and its chemical -104 properties. To establish upper and lower boundaries on the proton affinity of borazine it is necessary to ob1 2 !0 I 30 I 40 I 50 I I 60I ' 70I I 80 I 90 ! serve reactions in which protonated borazine is a pro6 ton donor and other reactions in which borazine is a Figure 4.-Variation in total energy (- --), total overlap popuproton acceptor. I n order to study borazine by chemilation (---), and overlap energy for trans-(NH3)PtC12(C2Ha) cal ionization methods, i t is essential that the carrier as a function of 6. gases are chemically inert with respect to borazine. Since the choice of such gaseous reagents appears to be Recent photoelectron spectroscopic studies of similar complexes lend partial support to our r e ~ u 1 t s . l ~ limited, we have devised a procedure involving the study of a three component gas mixture involving a The charge on the carbon atoms is estimated to be - (0.4 small quantity of borazine (B), a small quantity of a f 0.1) for the olefinic carbon and -(0.35 =k 0.1) for second proton acceptor (R), and an excess of inert carthe acetylenic carbon in (Ph3P)2Pt(C2H4) and (Ph3P)Zrier gas. Since a large number of ions are produced in Pt(CzPh2), respectively. I n comparison, population chemical ionization reactions when two hydrocarbons analyses determine a computed charge for the olefinic are present in a gas mixture, it is advantageous for purcarbon of -0.20 to -0.45 (Lowdin) or -0.74 to -0.84 poses of establishing reaction sequences to use partially (Mulliken) and for the acetylenic carbon of -0.50 to or totally deuterated compounds for one or two of the - 0.66 (Lowdin) or - 1.05 to - 1.23 (Mulliken) as the components. Two classes of reactions investigated angles 6 and w are varied. Greater negative charge were (1) reactions of a proton donor with borazine on the acetylenic carbon than on the ethylenic carbon through the sequence seems more reasonable since acetylene is a better A acceptor than ethylene, and acetylenes also have a CHs+ R ----t R H + CHd greater versatility for participating in the proposed RH+ B -+-BH+ R synergic mechanism. and ( 2 ) reactions of protonated borazine with a proton Since the structure of the molecule was kept rigid acceptor through the sequence while the angle 6 or w was varied, the variation of overlap population with 6 or w may be attributed to popCHs+ + B +BH' CHa ulation variations analogous to that of Blizzard and BH+ + R + R H + +B Santry.2 Hence the population of certain orbitals These reactions involve simple transfer of a proton bedefinitely affects the equilibrium geometry for free or tween stable molecules and therefore no competing complexed acetylenes and olefins. From previous process is usually possible. Haney and Franklin2 results4s5 and calculations by other methods2 we are postulated that the nonoccurrence of reactions of this inclined to ascribe this phenomenon to population of type is sufficient evidence of their endothermicity. J
-;i
w oL
+ +
p 5
+ +
1
+
(13) C. D. Cook, K. Y. Wan, U. Gelius, K . Hamrin, G. Johannsson, E . Olsson, H. Siegbahn, C. Nordling, and K .Siegbahn, J. Ameu. Chem. Soc., 93,
1904 (1971).
(1) R.F.Porter and J , J. Solomon, J . Amer. Chem. Soc., 93, 56 (1971). (2) M.A. Haney and J. L. Franklin, J . Phys. Chem., 73,4329 (1969).
Inorganic Chemistry, Vol. 11, No. 2, 1972 425
NOTES
TABLE I REACTION SEQUENCES INVOLVING BORAZINE AND PROTONATED BOIZAZINE Reaction mixture
Reaction composition
Pressure range, m m
Reaction sequence
+ H3BaN3H3
Pertinent observations
PA(HsBsNaHn), kcal/mol
>148 f l a or Occurs >173 f I* extensively Occurs >178 f 0.02 CaHa' HsBaNsH3 --* H3BaN3Ha' 460: 1 CzHe-HaBaNaHa Occurs CH5' C-CaDe 4 CaDeH' 70: 1.0:0.3 0.28-0.50 CH4-c-CaDe-HaBaNsHs extensively Occurs CH5+ HaBaNaHa --.c HsBsNsHa' extensively Probably ocCsDsHt HaBaN3H3 -+ HsBsNaHsD+ curs but competes with CeDizH+ Probably >167 f loor occurs >186 f Id Occurs >186 f le 182: 1 0.25-0.51 Occurs 146: 1.0:2.8 0.56 extensively Occurs extensively Shows no >189 f 1 reaction Occurs 100: 1.0: 1.5 0.26-0.54 extensively Occurs extensively Shows no >183 f 3f reaction Occurs 0.2-0.6 111:0.8: 1.0 extensively Occurs extensively Shows no >198 i 2 p reaction Occurs 103:1.0:0.7 0.32-0.51 extensively Occurs extensively Observed
