1898
NOTES
Scheme I11
-
+OH
II R-O-C-NHCHa 0
II
R-0-C-NHCH,
D
Hao/+
\\
It
R-O-C-N-CH3
OH
I +
+ I
R-o=c--"cH~
R-O-C=NHCH~
E 0-
H
0
.
OH
+/
+
H
+/
w R-O=C-N-CH3
\
\
H
H O
I
H
F
I II
R-O-C-NH-CH3
+
G energy of activation of the carbamate ester is lower (8.56 0.55 kcal mol-') compared to the amide (10.66 0.46 kcal mol-') offering some further evidence that carbamate esters exchange protons with greater facility than amides.
*
*
Summary and Conclusion From the data, carbamate esters and amides, in the presence of H30+, exchange protons through similar intermediates, but the rates of exchange are significantly different with carbamates exchanging faster than amides. This difference seems best explained as being due to differences in resonance and inductive influences by the two types of neighboring groups on the nitrogen basicities. Under the conditions of this study the carbamate nitrogen thus appears to be more basic than the amide nitrogen. These same influences appear to be of less consequence during base-catalyzed exchange as evidenced by identical rate constants for the amide and carbamate ester.
The Observation of Positive Temperature Coefficients in the Bromine Nuclear Quadrupole Resonance Spectra of the Diethylammonium Salts of Hexabromoantimony(II1) and Hexabromobismuth( 111)
by T. B. Brill* Department of Chemistry, Uniwrsity of Delaware, Newark, Delaware 19711
and G. G. Long Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27607 (Receiced January 11, 1971) Publication costs assisted by the University of Delaware
The halogen nuclear quadrupole resonance spectra of hexahalometallate ions are of interest because nuThe Journal of Physical Chemistry, Vol. 76, No. 12, 1972
merous possible variations in the metal halogen bond are detectable simply by changing the metal, while most other factors remain essentially constant.lI2 The present work was initiated in an attempt to extend the series of MXen- ions for which nqr data are known to Sb(II1) and Bi(II1). It was found that strong 7gBr and 81Br resonances could be recorded in [(CZH~)ZXHZ]BS~B and ~ G[ ( C Z H ~ ) Z N H ~ ] ~IB n ~addition, B ~ ~ . it was of interest to determine the temperature dependence of the halogen resonance frequencies for the purpose of comparison with other ;\fXen- ions.
Experimental Section Synthesis. The antimony (bismuth) salt was prepared by adding 5 g of SbzO3(Bi2O3)to 20 ml of hot concentrated HBr or HCI. After the Sbp03(Biz03)had dissolved, a stoichiometric amount of diethylamine was added dropwise to the hot solution, and upon cooling, the well-formed colorless crystals of the compound were filtered off and dried over KOH. Other compounds mentioned in this paper were synthesized and analyzed in a similar manner. Carbon and hydrogen analyses were carried out by Galbraith Labs., Knoxville, Tenn. Anal. Calcd for [(CzH5)zNHz]3SbBre: C, 17.49; H, 4.42; Sb, 14.78. Found: C, 17.75; H I 4.56; Sb, 14.96; mp 169-170". Calcd for [(CzH5)zNH2]3BiBrs:C, 15.83; H, 3.95. Found: C, 15.84; H, 3.79; mp 194195". Spectral Measurements. The nqr spectra were recorded using a Wilks Scientific NQR-1A spectrometer. Frequency measurements were made by zero-beating an external CW signal generator with the oscillator spectrum on a Tektronix 1L20 spectrum analyzer. The signal generator frequency was then measured precisely with a Nonsanto 150A electronic counter. Using superregenerative techniques, two closely spaced reso(1) N . Kubo and D. Nakamura, Advan. Inorg. Radiochem., 8 , 257 (1966), and references therein. (2) T. L. Brown and L. G. Kent, J . Phys. Chem., 74, 3672 (1970).
1899
NOTES nances cannot be separated. Reasonably symmetric resonance multiplets were detected at all temperatures implying that distortions in the anions, although probably present, are minor. Due to the difficulty of determining the center line of the resonance multiplet, the absolute error in the frequencies can be about 0.1 MHz. However, if what appears to be the same line of the multiplet is measured at all temperatures studied, then the relative error in frequency can be much less (1 part in 5000). I n Table I the frequencies are reported more accurately than the absolute error justifies because it is the relative differences that affect the results presented here.
Results and Discussion
The number of different cations which form hexahaloantimony(II1) and bismuth(II1) salts is somewhat more limited than with other metals because of the frequent formation of polynuclear complexes having widely varying stoichiometries. Simple counterions, such as K f , Rb+, and NH4+, which have been used in nqr studies of other hexahalometallates, frequently do not form C3MX6compounds with Sb(II1) and Bi(II1) under normal conditions. It is usually necessary to resort to alkyl-substituted ammonium ions for this purpose. Unsuccessful attempts were made here to record room temperature spectra in [ (CzH.&NHz JaBiCle, [ (n-C4H&NHzI3BiBre, and [ (CH3)zNHzl3BiBrg. However, [ (CzHa)&HZl3SbBr6 and [(CzH~)zNHz]3BiBr6 produced Table I: Bromine Resonance Frequencies (MHz) in strong 79,s1Br signals and their frequencies are shown in Diethylammonium Salts of Hexabromoantimony(II1) Table I. X-Ray powder diffraction patterns of these and -bismuth(III) a t Various Temperatures" two compounds and their ir and Raman spectra show a T, very good correlation between all the major peaks and it Compd OK u(19Br) S:Nb is reasonable to assume that they are isostructural. [(CzHdzNHzIaSbBr6 330 126.10 25: 1 Electronic Egects. Sb(II1) and Bi(II1) in MXe3- ions 298 126.190 20:l have valency shells containing seven electron pairs. 275 126.24 20: 1 If the seventh pair of electrons is stereochemically ac243 126.06 15: 1 tive, the R/IXe3- ion would not have regular octahedral 223 125.94 7: 1 193 125.60 3: 1 g e ~ m e t r y . ~The fact that only one nqr signal is found in both compounds indicates that ligand-ligand repulT, sions are more important than electron pair repulsion OK v(81Br) S:Nb and the extra electron pair has become inert. X-Ray [(CZH&NHZ] &Bre 330 87.74 18: 1 crystallographic studies are available for (NHJ2Sb298 87.93d 20: 1 Brs which contains S b B r P ions6 and [ (CH3)2NHz]3Bi275 88.02 17: 1 Bresand in both cases the anion is a regular octahedron. 243 88,07 14: 1 223 88.01 10: 1 It must be kept in mind that the nqr technique "views" 193 87.95 7: 1 the bromine atoms in their averaged environment and 77 87.65 10: 1 that molecular rearrangements are expected to be on a See Experimental Section for error estimates. b The filling about the same time scale as the nqr event.7 factor of the radiofrequency coil is significantly less during temComparison of the nuclear quadrupole resonance data perature dependence work so that the signal-to-noise ratios are for these SbBre3- and BiBra3- salts is valid since the significantly decreased. At room temperature with the highest two S ~ B ~ ~ compounds are isostructural. Table I shows that filling factor the S : N ratio for v(79Br) in [ ( C Z H & N H ~ ] ~ is there is a very marked difference between the bromine 6 5 : l and v(8lBr) for [ ( C Z H ~ ) Z N H Z ] is ~ B6 ~0 :Bl .~ ~c v(81Br) is 105.4 MHz. d v(79Br) is 105.5 MHz. signals in the two, and the increasing metallic character in going from Sb to Bi is readily reflected as an increase in the ionic character of the 34-Br bond. Comparisons The line shape of the resonances is, in part, a function of the SbBra3- and BiBre3- data with other hexahaloof differences in the oscillator operation over the various metal salts with different cations are tenuous because frequency ranges. The ranges of 120-130 RfHz and the resonance frequencies undoubtedly do not reflect 80-96 bIHz yield better line shapes than the 105-MHz solely the effects of changing the central metal. Variaregion. For this reason we chose to study v(81Br)in the tions of the cation would be expected to depress or amcase of bismuth and V ( ~ ~for B antimony. ~) Temperaplify the extent of the observed effects. ture variations were obtained by placing a Dewar cold Temperatwe Dependence Data. Figure 1 shows the finger containing the sample and cooling mixture in the effect of temperature variation between 360 and 190°K radiofrequency coil of the spectrometer. The temperaon the 79Br frequency in [(CzHa)2XH2]3SbBr6. The tures in Table I were obtained by the following means: warm HzO (33OoK), ice HzO (275"K), o-xylene (80%)(3) A . M. Phipps and D. N. Hume, J . Chem. Educ., 45, 664 (1968). (4) R. J. Gillespie, ibid., 47, 18 (1970), and references therein. m-xylene(20%)-Dry Ice (243"K), o-xyleue (50%)-m( 5 ) S. L. Lawton and R. A. Jacobsen, Inorg. Chem., 5 , 743 (1966). xylene (50%)-Dry Ice (223°K),aDry Ice and acetone (6) W. G. McPherson and E. A. Meyers, J . Phys. Chem., 72, 3117 (193"K), liquid Nz (77°K). The temperatures are (1968). reliable within 3°K. (7) E. L. Muetterties, Inorg. Chem., 4, 769 (1965).
*
The Journal of Physical Chemistry, Vol. 76, No. 18,1971
1900
NOTES
126.3126.2126.11260-
2
.-
125.9-
e
@ 125.8Y
J
'25*7i I
1256.-
150 200 250 330 350
T,
O K
Figure 1. The temperature dependence of the 79Br resonance frequency in [ ( C ~ H & N H ~ ] ~ S ~ B ~ B .
cients in hexahalometallates. Kubo and Nahamura and their collaborators' initiated much of this study. Armstrong and his coworkers8 have carefully investigated the potassium salts of PtCls2-, PdCls2-, IrCla2-, and OsCls2-. O'Learyg investigated K2ReC16. Brown and Kent2 reported considerably more data in many C,nilX~ systems. Among the transition metals with d orbital "holes," positive temperature coefficients in the halogen spectra are best accounted for using a mechanism involving the destruction of metal-halogen T bonding as a function of temperature.1J!8110 Negative temperature coefficients are by far the most common and appear t,o be related to low-frequency torsional modes of the a n i ~ n . ~ ~The ~ ~ "cubic ~ ' ~ antifluorite structure, which exists in most of the compounds in this past work, cannot exist in the (CZH~)~T\;H~+ salts. I n addition, little Sb-Br and Bi-Br T bonding is likely so that the mechanism causing the reversal in sign of ( d v / b T ) , in our compounds is not at all certain. It is evident, however, that phase transitions must be considered as being important in determining the sign of (bv/bT),.9 The resonance frequency can be thought of a t constant pressure as being a function of volume and temperature, v = f(V,T),so that*2n13
+
50
100
150
T,
200
250
300
350
O K
Figure 2. The temperature dependence of the *1Br resonance frequency in [(C2H;)eNH2]8BiBre.
sign of the temperature coefficient, (dv/dT),, is negative around room temperature but a phase transition takes place a t about 265" rt 4°K in this compound below which the sign of (bv/bT). becomes positive. The signal gradually becomes weaker as the temperature is decreased and was eventually lost in the noise near the Dry Ice-acetone temperature. No signals were found at 77°K. The same pattern for (bv/dT). is found in [(C2H6)2NH2]3BiBrs as is shown in Figure 2. The phase transition in this compound occurs around 255 4"K, and the resonance frequency remains detectable (although somewhat weaker) down to 77'1~ where measurements were ceased. In both compounds the phase transition is nondestructive. I n recent years much work has appeared related to the interpretation of temperature and pressure coeffiThe Journal of Physical Chemistry, Vol. '76, No. 1!2?19'71
(bvlbT). = (bv/bT). (bv/bV).(bV/dT). (1) Bayer'l has shown that (bv/bT) is normally negative. (dV/dT), is the important term in thermal expansivity and is positive. If ( b ~ / b Vis)assumed ~ to be positive,' the sign of (bv/bT), depends on whether the former or the latter term on the right side of (1) is larger. The volume of the unit cell is crucially dependent on phase transitions so that the latter term could be expected to change markedly in magnitude during the course of a phase transition. The change appears to be significant enough to reverse the sign of (bv/bT), in these compounds. This phase transition explanation is probably not "microscopic" in the sense that lowfrequency torsional modes and metal-halogen T bonding are. The onset of a phase transition simply provides a means for the latter two mechanisms to operate under different conditions. AcknowZedgment. The preparative skill of Ah. P. E. Garrou is gratefully acknowledged. (8) R. L. Armstrong, G. L. Baker, and K. R. Jeffrey, Phus. Res.$, 1, 2847 (1970); R . L. Armstrong and G. L. Baker, Can. J . Phys., 48, 2411 (1970); G. L. Baker and R. L. Armstrong, ibid., 48, 1649 (1970); R. L. Armstrong and D. F. Cooke, i b i d . , 47, 2168 (1969); R. L. Armstrong and K . R. Jeffrey, ibid., 47, 1095 (1969); K. R . Jeffrey and R. L. Armstrong, Phys. Rev., 174, 359 (1968); K. R . Jeffrey, R. L. Armstrong, and K. E. Kisman, ibid., B , 1, 3770 (1970). (9) G. P. O'Leary, Phys. Rev.Lett., 23, 782 (1969). (10) T. E. Haas and E. P. Marram, J. Chem. Phys., 43, 3985 (1965). (11) H . Bayer, Z. Phys., 130, 227 (1951). (12) T. Kushida, G. B. Benedek, and N. Bloembergen, Phya. Rev., 104, 1364 (1956). (13) H. S. Gutowsky and G. A. Williams, ibid., 105, 464 (1987).
