THE300-mp BANDOF N03-
2645
8.4 e.v. Thus, for a G(X) given by expression 5 we obtain (0.45 X 4.2) (0.4 X 8.4) = 5.3 e.v. Since the cyclization with its associated loss of double bonds requires no additional energy beyond that involved in the formation of the -CH2-CH=CH--cHradicals (the processes
+
-CH2-CH=CH-CH2-
+ H. + -CH~-CH--CHTCH~-
and
+
-CH2--CH=CH-CHy
+ Ha + -CH2-CH-CH-CH2-
+
being exothermic), we find that the minimum energy utilized in bringing about the various radiation chemical effects in polybutadiene is 34.6 5.3,or 39.9 e.v./ 100 e.v. absorbed by the polymer. This result implies a highly efficient utilization of the energy imparted to the system by ionizing radiation, inasmuch as the energy is not deposited, of course, in quanta of just 3.2 or 4.2 e.v., but instead over a range of 10-15 e.v. on down.
+
Acknowledgment. The author is indebted to Stanford Research Institute for supporting the present study.
The 300-mp Band of NO,-
by Eitan Rotlevi and Avner Treinin Department o j P h y s h d Chemistry, The Hebrew University, Jerusalem, Israel
(Received February 192, 1966)
The solvent effect on the low intensity ultraviolet band of Nos- was investigated. I n general, the oscillator strength of the band decreases with the polarity of the solvent, but its value for the free ion is not likely to be considerably smaller than f 7 X The assignment of the electronic transition is discussed. The forbidden g* + n transition appears to account for the properties of this band.
-
The weak absorption band of Nos- at about 300 mp has been the subject of many papers, and it was usually assigned to the lA1” + ‘AI’ ( r * + n) electronic transition. This transition should be extremely weak as it requires the coupling with at least two normal vibrations to make it appear. Since in crystals and solution the oscillator strength f of the band is lo2lo3 times larger than that calculated, it was suggested that ext,ernal perturbations lead to a large hyperchromic effect.l T o support this, it was shown that by varying the solvent from N,N-dimethylformamide to water f changes from 8 X lom5to 15 X 10-5.1 The few solvents tested were strongly polar, and therefore it was argued that the intensity would further decrease with decrease of polarity.1 To test this hypothesis we
carried out a detailed investigation of the spectrum of tetraalkylammonium nitrates in various solvents. The choice of the proper cation is decisive in solvents of low polarity a t relatively high concentrations (lo-“ 10-1 M ) , where formation of ion pairs may lead to distinct spectroscopic effects. The polarizing effect of the cations is responsible for the blue shifts observed in some previous works2 on decreasing the polarity of the solvent. Apart from the work of Strickler and Kasha there is only one early work,%where this effect (1) 8. J. Strickler and M. Kasha, “Molecular Orbitals in Chemistry, Physics and Biology,” Academic Press, New York, N. Y.,1964, p. 241. (2) (a) H. v. Halban and J. Eisenbrand, 2. physik. C h m . , A132, 401 (1928); (b) L. I. Katsin, J . C h m . Phys., 18, 789 (1950).
Volume 69,Number 8 August 1966
2646
EITANROTLEVIAND AVNERTREININ
was eliminated by using tetraethyl- and tetrapropylammonium nitrates. The solvents tested in that work were HzO, CHSOH, and CHC13.
r
I
I
I
I
I 300
310
I
I
Experimental Materials. Tetraethyl- and tetrabutylammonium nitrates were prepared by accurately neutralizing aqueous solutions of the corresponding hydroxides with nitric acid and then evaporating to dryness under vacuum a t 50". NEtbN03 and NBudN03 were recrystallized from acetone and ethyl acetate, respectivelyJ4 rinsed with acetone, and dried over CaC12. The purity of the salts was checked by measuring their spectra in aqueous solution, which were found to be nearly identical with that of KN03. The solvents used were of purest grade available. Diethylene glycol dimethyl ether (Ansul) and tetrahydrofuran (Fluka) were dried over sodium for several days, then refluxed with LiA1H4,and distilled. Absorption Spectra. The measurements were carried out with a Hilger Uvispek spectrophotometer. Solutions 0.05-0.1 M were used in 1-cm. silica cells. In the following solvents the spectra of both iYEt4NOa and ?;Bu4;\;03 were measured : water, ethanol, methyl cyanide, S,X-dimethylformamide, methanol, 2-propanol, ethylene glycol, dimethyl sulfoxide, and chloroform. No effect of cation was observed. Moreover, in the first four solvents the values obtained for hv,,, and emax are in good agreement with that obtained for lo-* M tetramethylammonium nitrate.l For all other solvents the butyl salt was used. Beer's law was also checked in CHCla and CHSCN, by varying M , and was found the concentration from 10-1 to to be valid. These results indicate that with these bulky cations ionic association (which may take place in the less polar solvents) has hardly any spectroscopic effect.
Results Figure 1 shows the absorption spectrum of NOa- in all the solvents tested. The position of the peak and its extinction coefficient are recorded in Table I together with some properties of the solvents which reflect their polarity. The general blue shift of the band with increase of polarity is evident. With few deviations the band gains intensity as it shifts to higher energies. The solvent effect appears to be mainly due to hydrogen bonding. (CHCb is known to have some protic pr0perties.j) Altogether, on lowering the polarity, the band reaches shape and intensity (f 7 X lo+) which vary only little with the nature of the solvent. Some vibrational structure is revealed in the aprotic solvents. It is badly resolved; the vibrational spacing appears to be constant a t -700 cm.-l.
-
The Journal of Physical Chemistry
I
280
290
I
I
320 A. m p
I
330
Figure 1. The absorption spectrum of NO8- in various solvents. The solvents numbered are (1) HO(CH&OH, (2) C~H~O(CHZ)~O(CH~)*OH, (3) CzHsCN, ( 4 ) (CH8)2SO, (5) CHsCOzCH, ('6)and (7) tetrahydrofuran and diethylene glycol-dimethyl ether, respectively (in these solvents the spectra are nearly identical).
Figure 2 s h o w the spectrum of X03- in mixtures of methanol with acetonitrile. There is evidence for an isosbestic point.
Discussion
-
Our results indicate that the intensity of the 300-mp band is not likely to be considerably lower than f 7 X loF5. Tiiis appears to be in discord with the lA1' transition; assignment of the 300 mp to lA1" though the intrinsic intensity is low, it is still too large for this type of transition. Sayre carried out a detailed analysis of the vibrational structure displayed by the weak b m d of crystalline nitrates a t 4°K.6 It is primarily coinpounded of vibrational frequencies of about 650 and 260 cm.-l.' The (0-0) band was -+
~
~~
(3) H. v. Halban md J. Eisenbrand, 2. physik. Chem., A146, 294 (1930). (4) The recwstalli.sation of tetrabutylammonium nitrate is difficult because of its high solubility in many solvents. Benzene should be avoided as it forms a complex with the salt (T. J. Plati and E. G. Taylor, J . Phys. Claem., 68, 3426 (1964)). (5) See, e a , C. M. Huggins, G. C. Pimentel, and J. N. Shoolery, J . Chem. Phys., 23, 806, 1244 (1955). (6) E. V. Sayre, ibid., 31, 73 (1959).
THE300-mp BANDO F Nos-
2647
Table I : Effect of Solvent on the Low Energy Band of NO3- a t 20’ Dielectric Dipole M-1 conmo2 cm.-1 stantb men3 vduec ern=,
XmaX,”
Solvent
mu
301.5 HzO HO(CHz)20H 302.5 CHsOH 303.0 CzHhOH 303.4 305.2 (CHs)2CHOH 306.6 (CH3)aCOH CsH~O(CHz)zO(CHz)zOH 306.7 CHCls 308.7 CzHsCN 310.5 311.2 CHzClz CHpCN 311.6 CHpC02CHp 311.5 312.0 Dioxane 312.5 CH,COzCzHh 312.6 (CHS)~SO 313.0 Tetrahydrofuran 313.0 CH~O(CH~)ZO(CHZ)ZOCH~ 313.2 HCON(CH8)z
7.44 7.37 6.83 6.36 5.75 4.19 5.85 6.77 4.65 5.55 4.83 4.09 3.76 4.01 4.42 3.36 3.36 4.01
80 39 33 25 19 13
1.8 2.2 1.7 1.7 1.7 1.7
94.6 85.1 83.6 79.6 76.3 71.3
4.8 27 9.1 37
1 . 0 63.2 4.0 1 . 6 64.2 3.4d 71.3 8.0 1 . 7 2.3 0 6.0 1.8 4.3d 71.1 7.6 1 . 7
37.6
3.8d 68.5
hms, was determined by the method of meana (G. Scheibe, Ber., 58,586 (1925)). In most cases the means were found to lie Unless otherwise stated the values closely on a straight line. of dielectric constant and dipole moment were taken from LanE. M. Kosower, dolt-Bornstein, Parts 61 and 311, respectively. J. Am. Chem. SOC.,80, 3253 (1958). A. J. Parker, &uart. Rev. (London), 16, 163 (1962).
’
not observed. All the vibrational bands which appear with relatively strong intensity are polarized in the plane of the ion. The substitution of N16 for N14 changes the frequency of the 650 cm.-’ value by about 2.5% whereas the 260 cm.-l frequency is not affected. A splitting of the vibrational structure is shown by KN03 but not by NaN08. Sayre pointed out that the combination of E’ electronic state with the two e’ vibrational modes is consistent with these results. He postulated a large expansion of the ion on excitation in order to explain the relatively large difference in the e’ frequencies between the ground and excited states. However, the lE’ + lA1’ transitions are allowed and so cannot account for this weak band. The two lowest E’ states are (n,u*) and (T,T*)states, in the m.0. treatment. The T* + T transition should be strong, and actually the 200-mp band has been The allowed u* + n transition should assigned to be rather weak but not as weak as the 300-mp band, and moreover its intensity should not be much affected by the medium. We are thus led to the conclusion t,hat, although the excited vibronic levels are of E‘
I
290
I
300
I
I
310
320 A,
mp
Figure 2. The spectrum of NOa- in mixtures of acetonitrile with methanol. (The volume per cent of methanol is recorded.)
symmetry, the corresponding electronic state belongs to another representation. If we accept Sayre’s vibrational analysis, then the appearance of the e’ progressions in the spectrum indicates that the ion is planar in its excited state. According to Walsh’s correlation diagram9 the lowest states that lead to planar configuration (and correspond to forbidden transitions) are (n,a*)Az’and (r,u*)E”. The forbidden u* n transition can be made to appear by coupling with a vibration of e’ symmetry. All the overtones of this vibration belong to reducible representations which contain the e’ representation.10 Thus, all the vibronic levels which belong t o the +
(7) The latter frequency was completely overlooked by Strickler and Kasha.1 (8) D. Meyeratein and A. Treinin, TTUW.Faraday SOC., 57, 2104
(1961). 2301 (1963). (9) A. D.Walsh, J. Chem. SOC., (10) The symmetries of the overtones of a degenerate vibration are clearly discussed by E;,B. Wilson, J. C. Decius, and P. C. Cross, “Molecular Vibrations, McGraw-Hill Book Co., Ino., New York, N. Y.,p. 362.
Volume 69,Number 8 August 1966
2648
electronic state A2‘ and vf = 1, 2 . . . should have the correct symmetry for an allowed transition polarized in the plane of the i0n.I’ However, the transitions from the ground state to these levels will be weak because the excited electronic state (A29 does not have the proper symmetry. If this picture is correct, then this case is quite different from that usually encountered when dealing with forbidden transitions which are relaxed by vibrational-electronic interaction (e.g., the 260-mp band of benzene). In the usual case it is the totally symmetric vibration which appears as a prominent progression, successive quanta of which are excited in combination with one quantum of a nontotally symmetric vibration. I n the present case the vibration which appears as a progression has in all its levels (v > 0) the proper symmetry to relax the forbidden character of the transition. I n both cases the common feature is the absence of the (0-0) band.12 The excitation of a nonbonding electron to a highly antibonding orbital (u*) is expected to bring about a considerable expansion of the ion, so that the observed reduction in the e’ vibration frequencies is not impr0bab1e.l~ McEwen pointed out14that the allowed u* + n transition in NOS- should be 1 or 2 e.v. lower than the corresponding transition in a less symmetrical compound such as nitromethane. The energy of the forbidden u* + n transition is probably even lower since it involves the highest filled n orbital. Thus, this transition may well appear a t 4.1 e.v. Leading to a considerable net flow of negative charge from the oxygen atoms to the nitrogen atom,15 the energy of this transition should be sensitive to environmental effects. However, since the vertical transition does not lead to a change in the dipole moment of the ion, the solvent effect is mainly due to the change in the H-bonding capacity of the ion on excitation. In this respect NOsdiffers from the pyramidal halate ions which exhibit a linear relation between hv and the 2 value of the solvent.16 The pronounced effect of the medium on the intensity of the band is probably due to its effect on the symmetry of the nuclear charge field.” Raman and infrared spectra of nitrates in crystals, melts, and solutionsl8 have revealed the distortion of the D3h symmetry exerted by polarizing cations. This effect seems to be in parallel with the intensification of the 300-mp band. I n some solvents of low polarity heavy
The J o u d of Physitd Chemietry
EITANROTLEVIAND AVNERTREININ
metal ions were shown to yield a large intensification of this band.2bg19 The effects of polar solvent molecules on the symmetry of NOs- are probably small. Thus, the splitting of the degenerate v3(e’) frequency could not be detected even in water.18 The largest hyperchromic effects exerted by solvents are due to protic solvents. This is probably due to the formation of definite (weak) compounds between solute and solvent (Figure 2). A small contribution to the intensification of the band in polar media may result from the decrease in the energy gap between the low and high intensity bands. The intensification results from the “mixing” of the corresponding excited electronic states induced by the proper vibration; the mixing coefficient is inversely proportional to the difference between the energies of the corresponding energy levels. However, the reduction of this gap is relatively small: -2% when replacing H2O by CH3CN,l which may yield a change of about 4% in the intensity. Finally, we have to consider the u* a (IE” + AI') transition. By coupling with one quantum of the out-of-plane vibration (a2”) the excited state gains the E’ symmetry. Provided the distortion from planarity is small, this quantum may be excited in combination with successive quanta of e’ vibrations. Since the a-orbital is actually a nonbonding orbita1,l the solvent effects can be readily explained. At present we cannot discard this assignment. +
(11) Strickler and Kasha ascribed the vibrational structure to the out-of-plane a2” vibration.1 In the absence of external perturbation only odd quanta of this vibration may appear. This results from their particular assignment of the band to an electronic transition which requires the coupling with a vibration of a2” symmetry. (12) This band may weakly appear in crystals owing to the perturbing effect of the medium. (13) The C=O stretching vibration in the (n,r*) state of some carbonyl compounds is 50-70% lower than in the ground state. In the (n, u*) state it should be even lower. (14) K. L. McEwen, J. Chem. Phys., 34, 547 (1961). (15) See ref. 1, Figure 2,for a schematic LCAO representation of the molecular orbitals of NOa- and a schematic energy level diagram. (16) A. Treinin and M. Yaacobi, J. Phys. Chem., 68, 2487 (1964). (17) C.F. Smith and C. R. Boston, J . Chem. Phys., 34, 1396 (1961). (18) For a summary of data and literature on these spectra see: L. I. Katein, J . Inorg. Nucl. Chem., 24, 245 (1962); S. C. Wait and G. J. Jane, QUCZT~. Em. (London), 17, 231 (1963). (19) C. C. Addison, B. J. Hathaway, N. Logan, and A. Walker, J . Chem. Soe., 4308 (1960).
