955
NOTES
The linear variation of ASo298 with the melting point quotient ( t M / i k ) is shown in Fig. 1 and can be represented by the equation
Table I : Melting Point Data6 and Entropy Data7 for Some Refractory Metal Carbides Melting point, Substance
Ti T i c (fcc) Zr ZrC (fcc) Hf HfC (fcc)
v
V2C (hcp) vc (fcc) Nb NbzC (hcp) NbC (fcc) Ta TazC (hcp) TaC (fcc) Cr Cr23C6 (fcc) Cr7Ca (hcp) CraCn (orth) MO
MozC (hcp)
MoC (hcp) W WzC (hcp) WC (hcp) a
O C .
1660 3180 1855 3480 2222 3890 1888 2165 2650 2467 3090 3500 3000 3500 4000 1915 1520 1780 1895 2620 2410 2575 3407 2800 2720
AS'zsa, tnr/tc
0.52
cal./deg. g.-atom C
-2.9 f 0.07
0.53
- 2 69 f 0.1'
0.57
...
0.87 0.71
-1.5 f O . l *
0.80 0.71
-1.6fO.l'
...
AX"298
=
-6.0
(1) This expression has been used to calculate values of tabulated in Table 11. Krikorian's estimates3 and recommended values are also presented in Table
11. Table I1 : Estimated Values of AS'ZSSfor Some Refractory Metal Carbides
... ...
0.86 0.75
- 1 . 6 f 0.3*
1.27 1.08 1.01
1.32 f 0.35 1.39f0.19 0.30f0.12
1.09 1.02
...
1.22 1.25
... ...
...
Obtained from the low temperature heat capacity data of
E. F. Westrum, Jr., and G. Feick reported in Tech. Doc. Rept. No. ASD-TDR-62-204, part 11, May, 1963. *Obtained from W. L. Worrell and J. Chipman, J. Phys. Chem., 68, 860 (1964).
of the refractory metal carbides would be from vibrational effects because changes in the electronic and configurational entropies are very small. A carbon atom is more loosely bonded in the carbide than in pure graphite; hence the lahtice vibrations and the vibrational entropy of a carbon atom should be greater in the carbide. However, A s 0 2 9 8 of a carbide is probably influenced more by the entropy changes of the metal atom because the vibrational entropy of a pure metal a t 298°K. is five to seven times that of carbon. A metal should be more rigidly bonded in a carbide whose melting point quotient ( t ~ l t c is ) less than one. The lattice vibrations and the vibrational entropy of the metal in this carbide would be less than that in the pure metal, and i\s0$?98 is probably negative. A similar argument would predict a positive AS"29e for a carbide which has a lower melting point than that of its parent metal ( t ~ / t C> 1).
-t G . O ( t ~ / t c )cal./deg. g.-atom of C
7 -
A S o m , cal./deg. g.-atom C -
This
Carbide
tM/tC
study
Krikoriana
Recommended
HfC NbzC
0.57 0.80 0.87 0.86 1.02 1.09 1.22 1.25
-2.6 -1.2 -0.8 -0.8 0.1 0.6 1.3 1.5
-4.0 f1.0 -1.0 f1 . 5 -2.0 f1.0 - 0 . 4 f 1.0 0.0 f 1 . 0 2.1 z t 1 . 0 2.5 f 1 . 5 0.6 f1.0
-3.0 f 1 . 5 - 1 . O f 1.0 - 1 , O f 1.0 -0.5 f 1.0 0.0 f1.0 1 . 5 f 1.0 2.0 f1 . 5 1 . O f 1.0
v2c
Ta2C MoC MozC WZC
wc
A precision of = k l O O o in the melting points of the carbides would result in a variation of approximately *lOyo in the calculated values of ASoZg8. However, because of the empirical nature of eq. I, no quantitative estimate of the uncertainties associated with the values of As0298 calculated in this study has been made. The uncertainties quoted for the recommended values in the last column of Table I1 are based arbitrarily on those of Krikorians and on the agreement between his estimates and those of this study. Acknowledgment. The author wishes to express his appreciation to Professor John Chipman for many helpful discussions.
The Nuclear Magnetic Resonance Spectra of Brornodiborane' by Donald F. Gaines and Riley Schaeffer Contribiition N o . 1178 from the Department of Chemistrg: Indiana University, Bloomington, Indiana (Received OCtobeT 2, 1963)
From studies of the aminodiboranes, Burg and Randolph2 postulated that the halogen atom occupied a bridge position in the halodiboranes. However, the Volume 68, Number 4
April, 1964
NOTES
956
preliminary microwave study of Cornwel13 could be interpreted only in terms of a terminal bromine in bromodiborane. This paper presents the IlB and lH n.m.r. spectra of bromodiborane and interprets them in terms of a terminal substituted model.
"i
Experimental Bromodiborarie was prepared and purified by the method of Schlesinger and Burg.4 The purified product had a vapor pressure of 40 mm. at -&', in good agreement with the literature. 4 , 5 Samples of the liquid for the n.m.r. spectra were contained in 5-mm. 0.d. Pyrex tubing. The llB and 'I-I spectra were obtained using a Varian Model 4300B high resolution spectrometer operating a t 19.3 and 60 ;\Ic./sec., respectively. To minimize decomposition, the temperature of the sample was maintained a t about -40' using the standard Vsrian variable temperature accessories. Results The IlB n.m.r. spectrum (Fig. 1) of bromodiborane can be interpreted only in terms of a terminal bromine. The boron to which the bromine is bonded is coupled to a single terminal hydrogen, giving rise to the large doublet at lower field. Each member of the doublet is further split into a triplet, with relative intensities of 1 : 2 : 1, by the two less strongly coupled bridge hydrogens. The other boron is coupled to two terminal hydrogens giving rise to a large triplet each member of which is also split into smaller triplets by the bridge hydrogens, as in diborane.6 The resonances of the two borons are partially overlapped, so that the central portion of the spectrum is a composite of the different resonances. The lH n.m.r. spectrum of bromodiborane also indicates two types of terminal hydrogens in a ratio of 2 : 1 (Fig. 2 ) . The peaks arising from one terminal hydrogen coupled to llB ( I = 3 / / 2 ) , marked a, occur a t lower field while those arising from two terniinal hydrogens coupled to llB, marked b, occur a t higher field.' Coupling of the bridge hydrogen with two different borons, using coupling constants obtained from the llB spectrum, results in a sixteen-lined group, marked c. Only the highest field members of the group are clearly visible, and they are partially overlapping. The contribution of loB coupling with the hydrogen has not been included in the derived spectrum. The chemical shifts for hydrogens coupled to 1°B mould be expected to be the same as for hydrogens coupled to ILB,?but the coupling constant is smaller by a factor of about three, and there are seven lines (the spin of loB is 3 ) instead of four. Thus the presence of 207, O 'B produces only a background, which is of little signifiT h e Journal of P h y s i d Chemistry
Figure 1. T h e
1lB
n.m.r. s p e c t r u m of bromodiborane.
n
e,
Figure 2.
The lH n.m.r. spectrum of bromodiborane
(1) Studies of Boranes X. For paper IX, see I. A. Ellis, D. F. Gaines, and R. Schaeffer, J . A m . Chem. Soc., 85, 3885 (1963). (2) A. B. Burg and C. L. Randolph, ibid., 71, 3451 (1949). (3) C. D. Cornwell, J . Chem. P h ~ s . 18, , 1118 (1950). (4) H. I. Schlesinger and A. B. Burg, J . A m . Chem. Soc., 53, 4321 (1931). (5) A. Stock, E. Kuss, and 0 . Priess, Ber., 47, 3115 (1914). (6) It. A. Ogg, Jr., J . Chem. Phys., 2 2 , 1933 (1954); J. N. Shooler\., Disctissions Faraday Soc., 19, 213 (1955): W. D. Phillips, H. C. Miller, and E. L. Muetterties, J . A m . Chem. Soc., 81, 4496 (1959): D. F. Gaines, I n o r g . Chem., 2 , 523 (1963). (7) I t has been found t h a t general chemical shift trends observed in '1B n.m.r. spectra cf boron hydrides are also observed in 'H n.m.r. spectra, though there is a t present no theoretical justification for this correlation.
957
NOTES
cance in terms of the over-all spectrum observed. Though the llB and lH n.m.r. spectra of brornodi-. borane contain regions where the overlap is sufficient, to render interpretation difficult, other regions are sufficiently free of overlap so that there is no doubt as to the assignments shown. Thus the structure of bromodiborane has been confirmed by an entirely independent method.
Table I : Chemical Shifts and Coupling Constants for the “R and lF1 n.m.r. Spectra of Bromodiborane H
\ /
\/H
/”\ llB
lH
6“ ( + 0 5), p,p,m. J (&f3), C.P.S. 6h ( 5 0 1), p,p,m. J ( + 3 ) , C.P.S.
Br
-18 9 163 -4 98 167
/El\H 2 141 -4 02 140
Br
\
/H\/
/B\
H
/”\
-12
56 4,c 44 2d - 1 2 (est.)
a With reference to BF,.0(C2H6)z. With reference to (CH3)4Si. c Bridge hydrogen coupling to the boron attached to bromine d Bridge hydrogen coupling to the boron attached to two terminal hydrogens.
Acknowledgment. This work has been supported hy a grant from the Kational Science Foundation. (8) D. F. Gaines, R. Schaeffer, and F. Tebbe, J . Phys. Chem., 67, 1937 (1963).
’
l i:
AgS03 Ag AgyOa KC1 KNO~-CS?~TO~ KSO~-CS??O~ (Lis(&) (LiXO?) 1
,
Association of’ Silver(1) and Chloride i n Molten Cesium Yitrate and i n Molten Mixtures of Potassium Nitrate with Cesium Nitrate or Lithium Nitrate’ by C. Thomas and J. Braunstein Department of Chemistry, Uniaersity of Maine; Orono, Maine (Received October $8. 2963)
(1) Supported by the U. S. .itoiiiic Energy Coinmission under Contract No. AT(30-1)-2873. (2) D. L. Maiming, It. C. Bansal, J. Braunstein, and XI. Blander, J . Am. Chem. SOC.,84, 2028 (1962).
The solvent effect on the association constants of (3) R. E. Haginan and J. Brauiistein, J . PhUs. Chem., 67, 3881 (1963). silver(1) with Chloride, bromide, or iodide i n the molten (4) D . L. Manning, M .Blander, and J. Braunstein, Inorg. Chem., 2, solvents XaN03 and K S 0 3 has been correlated with 345 (1963). the relative sizes of the cations and the relative sizes (5) J. Braunstein and A. S. Minano, ihid., 3 , 218 (1964). of the anions in terms of a ‘5-eciprocalcoulomb e f f e ~ t . ” ~ , 3 (6) (a) 31. Blander, F. F. Blankenship, and It. F. Newton, J . Phys. Chem., 63, 1259 (1959); (b) J. Brauiistein and 11. Blander, ibid., 64, The solvent effect of K a + or I
