April 5 , 1960
ELECTRON IMPACT STUDY
strong evidence of the occurrence of (1) in this gas phase radiolysis system. Since there is nothing unique about the reaction rate of ( l ) ,we conclude that these results support the view that ionmolecule reactions are of general importance in gas-phase radiation chemistry. An alternate possibility to the gaseous-ion sensitization is excited atom sensitization, because
BZ W
the quantity - should also be proportional to the rate of formation of excited species. Thus the mode of hydrogen atom formation might be X*
+ HZ--+ X f 2H
(19)
It is not possible to refute this completely, but i t would seem that if (19) were occurring to any great extent, then as the sensitizer pressure was increased, the probability of deactivation by X* X collisions would become more probable than (19). This would result in a non-linearity of R c I ~ # I I c ~ H ~ R Q H with , ~ sensitizer pressure, as has been observed for the rate of ethylene consumption as a function of argon pressure in studies of the ethyl-
+
+
+
[CONTRIBUTION FROM THE
O F THE
1555
CYANOGEN HALIDES
ene-argon systems5 The corresponding dissipation of charge in X + X collisions is not possible. It is not a t present possible to rule out the protonation of ethylene by XH+, viz.
+
XH' f CzH4 +X f CtHs'
(-SO)
However, the formation of C2H5+ would not be expected to lead to the same products as hydrogen atom formation in the presence of ethylene, unless H. This every C2Hs+ was neutralized to C2H4 latter event would, of course, be kinetically indistinguishable from (1) followed by neutralization of X H + and H3+. The reaction XH' CtHi --+ X ' f CsHj (21 1 can be ruled out on energetic grounds. Acknowledgments.-The author wishes to acknowledge fruitful discussions with Dr. J. H. Futrell, the assistance of Messrs. C. W. Bell and T. P. Gorman in carrying out the experimental work and the help of Mr. E. E. Poirot with the calculations.
+
+
BAYTOWN, TEXAS
MASSSPECTROMETRY SECTION, NATIONAL BUREAUO F STANDARDS]
Electron Impact Study of the Cyanogen Halides1 BY JOHN T. HERRONAND VERNON H. DIBELER RECEIVED AUGUST28, 1959 Mass spectra and appearance potentials are reported for the principal ions of cyanogen chloride, bromide and iodide. Using these data, the following thermochemical values have been computed: AHrCN = 89 =t 2 kcal./mole; 4HfCX- = 15 =t 4 kcal./mole, E.A.(CN) = 74 f 4 kcal./mole.
Introduction The cyanogen halides form an interesting series of compounds bearing certain resemblances to mixed halogen compounds in the ease with which they form negative ions. Furthermore, they provide a homologous series containing the CN group, of which the heat of formation, ionization potential and electron affinity are of interest for numerous thermochemical applications. Recent studies of the ionization-dissociation processes in cyanogen, C2N2, have been reported by McDowell and Warren,2 Kandel,3 and by Stevenson. * The only published appearance potential for the cyanogen halides is also due to S t e v e n ~ o nbut , ~ is limited to the C1+ ion of cyanoAs some uncertainties remain in the gen chloride. interpretation of the dissociation process in cyanogen chloride and cyanogen, further work including the halogenated series seems to be warranted. An additional incentive is the reliable determination of heats of formation of cyanogen chloride, bromide and iodide by Lord and W00lf.~ (1) This research was performed as part of the National Bureau of Standards' Free Radical Research Program supported by the Department of the Army. (2) C. A. McDowell and J. W. Warren, Trans. Faraday S O L . , 48,
1084 (1952). (3) R . J. Kandel, J . Chem. Phys., 22, 1496 (1954). (4) D. P. Stevenson, i b i d . , 18, 1347 (1950). ( 5 ) G.Lord and A. A. Woolf, J . Chem. Soc., 2546 (1954).
Experimental The mass spectrometer was essentially the conventional 60" sector-type instrument. Magnetic scanning was used a t a constant ion accelerating potential of 5000 volts. Mass spectra were obtained using 70 v. electrons. Usual procedures for appearance potential studies were employed. That is, the anode of the ion source was operated a t about the same potential as the ionization chamber. The ion repeller was biased 3 v. positive with respect to the ionization chamber when studying positive ions and 1.5 v. negative for negative ions. The temperature of the ion source was about 250'. Appearance potential measurements were made by varying the electron energy by means of a Shallcross decade potentiometer across a carefully regulated and measured 100.0 v. power supply. The electron accelerating voltage was usually varied in 0.1 v. steps. The electron energy scale for positive ions was calibrated by means of argon, using the spectroscopic ionization potential of 15.76 e.v.6 In the case of negative ions, no single calibration point was found acceptable over the required energy scale. I t was found necessary to separate negative ions into two energy ranges, i.e., 0 to 2 ev. and above 2 ev. Furthermore each range required an optimum setting of source magnet and emission control to attain stable operation. Ion currents were measured with a 10-stage electron multiplier, vibrating reed electrometer and recording potentiometer. Under favorable conditions, the minimum detectable current was estimated t o be lo-'* amp. Table I lists the negative ions used as calibration points. Positive ion appearance potentials were evaluated by normalizing the slopes of the unknown and reference curves, plotting on a semi-log scale and determining the voltage in( 6 ) C. E. Moore, N a f l . Bur. Slandards Circular, 467 (1949).
TABLE I NEGATIVE 10s CAIJBRATIONPOIXTS Molecule
Ion
CO
0-
Probable process
+ +
A . P . (er.)
11e t h ~ x l
CO(lZ+) + C+(*P) 0-(2P) 20.902 35 0 . 0 2 Calcd." 0 -. CO('Z+) + C(3P) 0--(2P) 9 . 6 zk(O.2 Elect , i r n pac t" 01 00 2 ( 3 2 g - ) + oLi4s)-t- 0('PI 11.264 f O.(JO(3 Calcd." 002(32g-) -+ o ( 3 ~ ) 0 - p ~ ) 4.53 It .03 K.p.tl. elect. impact" HCl c1HCl('Z+) + H(%) Cl-(*S) 0.62 .05 R.p.d. elect. impactd 0.0 1Z.p.d. elect. inipncte SF, SF6SFG+ SFsI . a L. M. Branscomb, J . Chem. Phys., 29, 452 (1958). H . D. Hagstrum, ibid., 23, 178 (1955). c D. C. Frost and C. . McDowell, THISJOURNAL, 80, 0183 (1958). R . E. Fox, J . Chem. Phys., 26, 1251 (1037). If.. hI. Hickmi a n d R. E . Fox, i b i d . , 2 5 , 642 (1956).
+ +
terval between the nearly linear portions of the normalized curves. Cyanogen chloride was prepared by the reaction of hydrochloric acid with a mixture of mercuric cyanide and potassium chlorate. The gas evolved mcts passed over solid potassium hydroxide, to remove water and unreacted hydrochloric acid. Cyanogen bromide was obtained from a commercial source, and cyanogen iodide was supplied through the courtesy of Dr. D . E. Mann, National Bureau of Standards. The cyanogen halides were shaken with mercury before use t o remove free halogen. The bromide and iodide compounds alscj were held a t - 70" and pumped for about onehalf hour to remove any volatile impurities. Mass spectral analyses showed these impurities: in CTc'Cl, HCN, 0.5$G, HC1 0 . 2 7 , , hydrocarbon 370'0; CNBr, HCN, 0.2%, HBr 0.270; CNI, HCN, 1.5%, CNBr, 0.5%. Several attempts to prepare CiYF by various means were uniformly unsuccessful. Although Cosslett' reported the preparation of cyanogen fluoride by the reaction of cyanogen iodide mith silver fluoride a t 220", all subsequent attempts to repeat this preparation have failed.* It now seems certain that Cosslett's product Tva? principally a mixture of COz and SiFd.
Results and Discussion Mass spectra of the cyanogen halides are given in Table 11. The lower masses of the CNCl ( m 'e < 20) are in some doubt due to the correction TABLE I1 MONOISOTOPIC MASSSPECTRA Ion
C+
N+ CN X' CX NX
cy-x s+-
CX 4- + CNX+ CXMax. XMax.
C?iCI
(2.36) 0.94 5.83 9.15 2.97 ..... 100,o
..... 0.03 1.03 0,003 ,004 ,001 ,002
CNBr
0.54 0.12 1.17 10.25 1.54 0.03 100.0 0.13 0.03 3.36 0.00
.76 ,0002 005
CNI
0.25 .04 .86 27,33 1.39 0.03 100.0 0.82 0.02 3.13 0,007 ,005 N ,0001 ,0001
-
for presence of a non-separable hydrocarbon impurity in the cyanogen chloride. The mass spectrum of cyanogen chloride differs somewhat from that reported by Stevenson. However, Stevenson's data were obtained a t 55" and 1000 v. ion acceleration voltage. Like the halogen molecules, the molecular ions are by far the most abundant. Fragment ions resulting from the rupture of the CN-X bond show interesting variations. The (7) V. E. Cosslett, Z . axoi'g. Chenz., 201, 75 (1031). ( 8 ) €1. J. Callomon, H. TI;. Thompson, I;. A. Anderson and R. Bak, .I. C h m . SOL. (Loiidon), 3709 (1953).
*
X + ion increases from 9.2c/, (sum of the isotopes) in CNCI, to 10.3 yo in CKBr, and 2i.;3YGin C S I , whereas the abundance of the CN' ion decreases from 7.670 in CNCl to 0.86% in CNI Apparently this is related to the difference in the ionization potential of the CN radical (14.6 ev.) and of the halogen atoms as the latter decreases from Cl (13.01 ev.) to I (10.45 ev.). Doubly charged ions of the molecule and of fragment ions are observed in each instance. There is some indication of the rearrangement ions, NX ?. Negative halogen ions and C S - ions are observed in each case. Kegative ion abundances are tabulated at 70 ev. except for the resonant processes which are reported for the apparent maximum cross sections for the dissociative attachment process and are related to the most abundant positive ton measured a t 70 ev. Table 111 summarizes the measured appearance potentials along with the postulated reaction mechanisms. Standard deviations are given for the positive ions. In the case of the negative ions, where the number of measurements was much smaller, the uncertainties have been estimated. Positive Ions. CNX+.-The molecular ionization potentials are reported here for the first time. These values are very nearly equal to the ionization potentials of the halogen atoms I(C1) = 13.0, I(Br) = 11.84 and I(1) = 10 45 ev and suggest that ionization occurs by removal of a nonbonding electron from the halogen atom i n the molecules. Xf.- Stevenson4 measured the Clf appearance potential from cyanogen chloride apd attributed I t to the process C R ' C l - - + CX + c1+, il P. = 17 81 c\ from
+
+
A(C1') = AHICY AHrCl' - AIITrcsc1 f Ee Ek where E, is any electronic excitation energy and E k e is the total excess kinetic energy possessed by the neutral and charged fragments, it follows that AHrCIY
5
P
.%(Cl-) f LHfCNC1 - AHrCI'
Substituting Stevenson's measured -4(Cl+) from Table I11 and the heats of formation from Table IV gives AHf(CN) 5 113 kcal./mole. Stevenson suggested that in this reaction the CN radical was in the A% electronic state ( E , = 2G.4 kcal. /moleg) If this is the case, the heat of (9) G Herzberg, "Spectra of Diatomic Molecules," D Van N o 5 t r i n d C o I n c Yew Ymk S Y I'l?O
ELECTRON IMPACT STUDY OF
April 5 , 1960
TI-IR
1557
CYANOGEN HALIDES
TABLE 111 APPEARANCE
POTENTIALS I N
(CN)n
-+
Other work Pos. ion (ev.) Neg. ion (ev.)
Neg. ion (ev.)
Pos. ion (ev.)
Process
CYAh’OGEN HALIDESAND CYANOGEN
This work
7
13.57 zk 0.02? 20.75 =k .043
(CN)?’
+ CN CN- + C N CNCl+ CC1+ + N C N + + C1 CN’ + C1C N + c1+ CN- + C1+ cx- + c1 c s + c1-
+ C” +
CSCl
+ + + . -.f
+ + .
+
+
?
+
11.95 17.4 18.3 14.6
+ + +
+ Br+ + Br+ CN- + Br C N + Br-
-,C N
+ CN+
+
? CKI
f 0.01 f .2 f .2 i .2 f .1 i .1
15.5 i .3 17.81 f. . Z 3 13.3 & 0.4 f 6.7 f -0 =k
c1-
C S B r -+ CNBrt +. CBr+ N + CNf Br +. C N + Br-
+Br+CNIf + CI+ N -+ C?;+ I + CN+ I-tCN+I+ +. CNI+ -+CN-+ I + .C N I-
=k
+ + +
16.2 zk .1 11 9 f .2
bromide >iodide. For the C X + ions of the cyanogen halides, this order is reversed. The ions must be formed by the simple process S = C X +cx+-t N The ionization potentials of the C X + ions can be calculated from
+
/l(C);') = D(sEcx') I (cx') f Ee f r o m which i (CX) 5 A ( C X + ) - D(X=CX)
+
Vol. 82
Ek.8.
The values calculated in this manner using a value of 160 kcal. 'mole for D(N=CX) are I(CC1) 5 10.2 ev.; I(CBr) 5 10.5 ev.; and I(C1) 5 10.8 ev. Within the extreme limits of estimated uncertainties in the measurement together with plausible small variations in D(C=N) due to the nature of halogen substitution, the observed I(CX) values may be considered very nearly equal. By direct measurement, Reed and Sneddon" found I(CC1) = 12.9 and I(CBr) = 10.4 ev. The latter is in good agreement with present results, but the value for IjCCl) is in serious disagreement. It is interesting to note that the above calculations result in upper limits to the ionization potential and that excess electronic or kinetic energy would result in smaller values for the ionization potentials than those indicated. Ion-pair Processes.-Several positive and negative ion appearance potentials have been identified as ion-pair processes. These are of special interest as there is as yet no experimental evidence to support the existence of excited states of negative atomic ions" and any excess energy of the products is likely to be kinetic energy. Because of the greater reliability of the positive ion energy scale and the fact that positive ion ap(10) I.. H r e w r , I.. K. Templeton and F.A. Jenkins, THISJOURNAL,
73, 1402 (10Z1). (11) R. I. Reed and W. Sneddan, Z'ralzs. I;araday Soc., 54, 301 (1958). (12) I,. M. Branscomb, "Negative Ions," Advances in Electronics rind Electron Physics. Academic Press, Inc., New York. N. Y., 1957.
A(CS+)
=
+c s ++ x-
A ( X - ) = AHrCX'
+ AHfX- - AHiCNX + Ee -I- E k e
hence AIIfCS+ 5 A ( C S + )
+ AHrCXX - A H i S -
Substitution of the measured appearance potentials from Table I11 and heats of formation from Table IV results in AfIfCNf 5 448 and 437 kcal./ mole for the chloride and bromide, respectively. These values are considerably greater than the lowest value calculated in a previous section for AIIfCN+of 424 kcal./mole and suggests that in the present case the appearance potentials contain either kinetic or electronic excitation energy terms of about 1 and 0.6 ev., respectively. In the case of cyanogen bromide, it was found that the Br- ionization efficiency curve had a resonance type maximum a t the foot of the curve, so that no unambiguous single onset energy could be determined. CN-, X+.--The process in this case is CNX -++ CNsand
+
il(CN-)
=
il(St) = AHrCS-
+ AHrX+ AHiCNX
hence
+ E, +
Ek.e.
+
AHiCN- _< A(X+) AHiCNX - AIliXt and AEIf CN- 5 16, lS, 11 kcal./mole for the chloride, bromide and iodide, respectively. The mean value from all determinations was AIIfCN- 5 15 f 4 kcal. ,'mole. The electron affinity of Ch' can be calculated directly from CS----t
CS
+ e-
for which AH = E.4(CN-) = AHfCK - AlIrCNhenceEA(CN-) = (S9 f 2 ) - (15 f 4) = 74 f G kcal./mole. The electron affinity of C Y also can be obtained from the processes CNX +C N C S S +C X -
from which CN-
+ CX
+ e - , AH
+ X+ + X+
AIfi
AH,
= EA(CN-) = AH1
-
AH2
Thus by taking differences between the measured X + appearance potentials for every individual experiment (assuming the neutral C N is produced in the A211 state), we calculate EA(CN-) = 74 4 kcal./mole (3.2 + 0.2 ev.). Bakulina and I O I I O Vmeasured ~~ the electron affinity of C N by surface ionization of KCN and KCNS on a hot tungsten wire. At 2210, 2230 and 2440°K., the values found were G9 to 74 kcal./
*
(13) I. N. Bakulina and N. I. Ionov, Doklady A k a d . Nauk S.S.S.R., 99, 1023 (1954).
HYDROGEN-BONDING AND SPECTRA OF BENZOIC ACIDS
April 5 , 1960
mole for KCN and 69 to 76 kcal. for KCNS. The mean value, 72 kcal./mole, is in good agreement with the present calculation; although the temperature dependence of their results creates some uncertainty. Negative Ions by Resonance Capture. X-.Figure 1 shows an ionization efficiency curve for the Br- ion from cyanogen bromide. The small resonance peak near zero volts may be due to a small HBr impurity. The main peak has an onset of 4.1 =k 0.3 ev. In the case of the iodide no peak was observed near zero volts, the main peak having an onset of 2.6 f 0.3 ev. On the other hand the chloride had a large peak a t or near zero volts and a second peak with an onset of 6.7 f 0.3 ev. We are not certain as to the origin of the first C1- maximum. If the process is represented by
csx ----f
CN
+ x-
@ I
2; ,-E
1
0
0
2
the appearance potential can be calculated from .. A(X-1
AHrCS
+ AHfX-
-
AHrCiYX
+ E, +
&.e.
For and E, zero, the values so obtained are -0.04, -0.43 and -0.43 ev. for the chloride, bromide and iodide respectively. This would indicate that the measured values contain from 2.2 to 6.7 ev. excess energy. CN-.-This ion is formed by the process CSX+CN-+X
The onset for this process is nearly zero volts for the bromide and iodide and about 0.4 ev. for the chloride. The appearance potential can be calculated from A ( C ? ; - ) = AHrCN-
+ AHrX - AHrCNX
assuming no excess energy is involved in the dissociation. The calculated values are 0.6, 0.0 and
1559
4
6
8
10
12
14
16
ELECTRON ENERGY, E.V. U N C O R R E C T E D .
Fig. 1.-Ionization
efficiency curve for Br- from CNBr.
-0.4 ev. for the chloride, bromide and iodide, respectively. The experimental and calculated values are sufficiently close to indicate that only relatively small amounts of excess kinetic energy can be involved in the dissociation. summary Appearance potential measurements of both positive and negative ions of the cyanogen halides lead to consistent values for the heat of formation of the CN radical of 89 kcal./mole and of the CN- ion of 15 kcal.,'mole. Thus the ionization potential of the CN radical is 337 kcal. (14.6 ev.) and the electron affinity is 74 kcal. (3.21 ev.). WASHINGTON, D. C.
[CONTRIBUTION FROM DEPARTMENT O F CHEMISTRY, FACULTY O F SCIENCE, KYUSHUUNIVERSITY]
Effect of Temperature on Ultraviolet Absorption Spectra of Benzoic Acids and its Relation to Hydrogen Bonding BY hfITSW0 I T O , HIDEOTSUKIOKA AND
SUNAO IMANISHI
RECEIVED AUGUST18. 1959 The near ultraviolet absorption spectra of benzoic acids in petroleum ether were measured a t various temperatures ranging from +lo0 to -180". At low temperatures absorption bands due to dimers appear a t wave lengths greater than those of monomer bands for all the substances investigated. Effect of hydrogen bonding on the absorption spectra of benzoic acid was also studied at low temperatures for the solution systems containing proton donating or accepting molecules. The results show that the direction of spectral shift due to hydrogen bond formation is determined whether benzoic acid behaves as a proton donor or a proton acceptor in hydrogen bonding. The observed spectral shifts are discussed qualitatively.
Introduction In a previous paper it was reported that absorption spectra of benzoic acids near 2800 A. show a remarkable effect of concentration in non-polar hydrocarbon solvents which was interpreted in terms of equilibria between monomer and dimer forms of solute molecules. Hydrogen bonding, through which benzoic acids associate into dimers, is in general a weak binding and its effect on electronic absorption spectra is usually small. T o be able to estimate such a small effect, the electronic (1)
M.Ito. J . Mol. Spectroscopy, in press
absorption bands must be sharp enough. Unfortunately, the absorption spegtrum of benzoic acid in the region near 2800 A. is considerably broadened and exhibits only a very vague vibrational structure even with a solution in a nonpolar solvent, contrary to the cases of other monosubstituted benzenes which exhibit generally sharp absorption bands. I n order to investigate this apparently small effect of hydrogen bonding on the absorption spectra of benzoic acids, measurements were made a t low temperatures by utilizing the fact that
