1312
C. BARTER, R. G. MEISENHEIMER ASD D. P. STEVENSON
Vol. G4
DIh31,4GNETIC SUSCEPTIBILITIES OF SIMPLE HYDROCARBONS 9 S D VOLATILE HYDRIDES BY C. BARTER,R. G. MEISENHEIMER AKD D. P. STEVEXSON Shell Development Company, Emeryville, California Receaced ApizZ 11, 1960
The upe of a Pauling Type oxygen meter in the measurement of the diamagnetic susceptibility of gases at subatmospheric pressure and ambient temperature is described. The method has been tested on the rare gases, and applied to measurements on nineteen light ( 5 C,) hydrocarbon gases, diborane, ammonia, silane, phosphine, germane, arsine, stibine and methyl fluoride. The results obtained on the various gases are compared with the results of previous measurements and/or values deduced by various extrapolation procedures from measurements on more complex substances in the liquid or solid state.
Introduction In connection with studies of the relation between the cross sections for ionization by electrons and the constitution and the structure of molecules’ it became of interest to compare the ionization cross sections with a molecular property directly related to the mean square electronic radius. Such a molecular property is the diamagnetic susceptibility.* Fa1103 has suggested that there should exist a proportionality between the diamagnetic susceptibilities of substances and their cross sections for ionization by high energy electrons. A search of the literature4 revealed the virtual absence of data suitable for comparing the constitutive and structuraldependence of diamagnetic susceptibilities and ionization cross sections. Although measurements of the diamagnetic susceptibilities have been reported for an extraordinary number of complex substances, there have not been reported results of measurements on simple, normally gaseous substances of the type particularly suitable for examining the constitution and structural dependence of these molecular properties. The work reported in the present paper eliminates in part this gap in the knowledge of the diamagnetic susceptibilities. Consideration of the very small magnitude of the effects associated with diamagnetism, quickly reveals why virtually no measurements of the diamagiietic snsceptibility of gases have been reported in the past. However, the development of the High Sensitivity (Pauling) Oxygen Meter by the A. 0. Bechian Company makes readily available an instrumeiit suitable for the quantitative conipnrimnof the diamagnetic susceptibility of‘gases even at subatmospheric pressures. In this paper we present the results of measurements of the diamagnetic susceptibilities of nineteen light (web. However, acetylene like cyclopropane hslq a significantly larger diamagnetic. 4usceptibility than that calculated from data on 1 3 ) AIethanr becomes n h f t h group of one member in the i n g u s . nl formulation 14) T h e datuni of I’inh a n d Ubhelohde (ref 1 . Table 11) on n iir\ane ~ r a not s inrluded because it appears t o b e greater than t h e more i r r r n t n r r r i w \ \ r i r L of Hroprsma and of 4 n m c rl ai h) ‘I siifirlpnt ainoiint t o ronrlii 11 tlidt a > x p t P n i a t i ( error o < r i i i i e d 1 1 1 tills e.tTllcr 11 rrrh 115) bee p a K c 6 12 a i d l a of ief 1. t
1
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TABLE 1V CONPARISON O F CYCLO.4LKlNE PROPERTIES WITH
TL-ALKANE
PROPERTIES Cycloa1k:ine
I ( - x - 11.68nI
Cyclopropane Cyclobutane Cyclopentane
+4.2(gas) - 6 . 7 (gas) - 2 . 2 (gas); $0.8 (liq.1 - 5 . 0 (liq.) - 7 . 9 (liq.) -8.1 (lis.)
AH,
[ E - 4.64ni1a c.IIsec.c
+0.08 - .32
+50 -24
- .06 0 Cyclohexane - .13 0 Cycloheptane - .30 -8 Cyclooctane - .46 -7 Cyclononane . . .. . ... .... - 6 Cyclodecane .... . ... - .91 -6 Cyclododecane - i .7 (Xtal) - .63b .. * T h e molar refraction R = (no2 - l)/’(no2 2) X 1.. 4.64 is the average CH2-increment for R (n-alkanes) from S.B.S. circular No. (3-461. * F o r this substance, niou 1.4536 and 0 7 0 4 0.8273 g./ml. were determined in our Analytical Dept. under the direction of W. R. Harp, Jr. c The nuclear magnetic resonance proton chemical shift from the corresponding n-alkane, measured at 40 Megacycles by D r . C. A. Reilly of these laboratories. Uncertainty of measurement, estimated t o be i 1 to i 2 r.;’sec.
+
more complex substances containing this part,icular st’ructuralunit. KO definitive conclusion can be reached with respect to the question of the effect of state of aggregation on the diamagnetic susceptibilit’yfrom t>he data in Tables I1 and 111. Considerably greater precision than we have been able to achieve in the measurements on gases would be required to assure t,hat the apparent effect, x gas I< ,x liq., is real. This is in agreement with earlier conclusions of Selwood.6 Cert’ainly the general quality of the agreement between our measurements for t’he lower hydrocarbons with the values calculat’ed from the group constants of Angus based on liquids, suggests that t,here is no real difference between gaseous and liquid values of this molecular quantit’y. There are summarized in Table V data on the diamagnetic susceptibilities of gaseous hydrides more or less isoelectronic with rare gases. For comparison with the experiment,al values shown in columns 2 and 3 of t,his Table V, there are given in columns 4 and 5 susceptibilities calculat’ed under the hypothesis that the quaiitit)y is a constitutive molecular property. In column 4 there are given the sums of the atomic susceptibilities per mole calculated from the Tan Vleck equation’l
where (q?}, is the 11itla11 q u a r e radius of the ith atomic. electroil, as e\duated for Slater electronic wave functions for the atoms.16 I n colrimn (i there are give11 the sums of the l’ascd for the atoms. It, appears that for elements of ally rom of the periodic system, the de\-iation frow additivity of the diamagnetic susceptibility iucreases with the distance of the “central atom” of the molecule from the rare gas atom. Further in the hydrides of any group of the periodic system the (16) See t o r example, J. 0. Hirschfeider, C. P‘. Ciirtiss a n d H. R. Bird. “Alolecular Theory of Gases and Liquids,” John R‘iley and Sons. The iise of Hart,ree-Yorb In?.. N e w York. N. Y..1954, p p . O.il-.9i.i. ?lwtronie n a i ~fiinctions for the atonis, see voliiinn > u t l’ahl? l’, niabes no significant cliange in the calculated 1 ‘ s .
C. BARTER, R. G. MEISENHEIMER AND D. P. STEVENSON
1316
TABLEV OF VOLATILE HYDRIDES AND DIAMAGXETIC SUSCEPTIBILITIES METHYLFLUORIDE - x X 106,c.g.s., emu.!moie
Slater
HartreeFockn
Pascal
........ 4.00 4.76 4.76 5.86 Hz 1.65 2.06 ... 2.02 f 0.08 1.90 He ... 17.9 19.8 16.0 '/PB~Hs 10.5 k . 4 l i . 4 f .8 12.2 18.7 20.5 17.7 CHa 15.0 17.0 14.3 16.3 f .8 16.5 NH3 ........ 13.0 (liq.) 11.7 13.8 10.5 HF ........ 8.6 8.68 11.0 9.2 CHBF ... 24.2 26.7 21.1 1 i . 8 f .8 Xe 5.64 8 . 3 0 ... 6.96 i .14 7.02 20.4 f .8 ... 34.7 24.7 SiH4 PHl ... 30.5 18.8 26.2 f .8 ........ 25.5 26.2 20.9 HzS ........ 22.1 22.3 23.0 HC1 18.5 21.1 ... (19.32) 19.32 Ar 29.7 Z k .8 ... 46.9 37.i GeH4 -4sH3 .8 ... 43.4 29.8 35.2 ........ ... 39.1 28.5 HzPe ........ 32.9 35.1 33.5 HBr Kr 28.G 31.2 31.9 ... 29.0 f .4 ........ ... 68.5 ... SnHd 34.6 f .8 ... 63.1 35.8 SbH, ........ ... 57.4 ... HzTe ........ 47.7 52.1 47.5 HI 47.1 ... 45.5 f .7 43.2 Xe See J. A. Ibers, Acta Cryst., 11, 1781 (1958), and B. K. Vainshtein and J. A. Ibers, Kristallogrujiya, 3,416 (1958).
*
deviations from additivity increase rapidly with increasing atomic number of the "central atom." In all cases it will be noted that the sense of the deviation from the sum of the atomic susceptibilities is the same, i.e. lYoba1
*)f.l.
