KOTES
562
glasses. To our kno rvledge no other compilation of this nature has appeared in the literature. We wish to thank Dr. R. 8. Becker for helpful suggestions and the use of the facilities of the Spectroscopy Laboratory. TABLE I Low TEMPERATURE RIGIDGLASSSYSTEMS System
Hydrocarbons 3-Methylpentane Isopentane :methylcyclohexane Pentene-2 (cis)-pentene-2 (trans)"
Composition (VOX. :vol.)
1:4
Alcohols Methanol :ethanol" Ethanolb Isopropyl alcoholb l-Propanolb 1-ButanoI' Ethers n-ButyI ether :isopropyl ether: diethyl ether 2-Methyltetrahydrofuran
1 :4
3:5:12
Alcohols: Ethers Ethanol: diethyl ether Propanol: diethyl ether Butano1:diethyl ether
1:1 2 :5 2:5
Alcohol: Ether :Hydrocarbon Ethanol: isopentane: diethyl ether Isopropyl alcohol :isopentane :diethyl ether
2:5:5 2:5:5
Hydrocarbons: Ethers Diethyl ether: isopentane 1:l Diethyl ether:pentene-2 (czs)-prntene-2 ( t ~ a n s )2: ~1 Miscellaneous Diethyl ether:ethanol: toluene 2:l:l Diethyl ether :isopentane :ethanol: dimethylformamide 12: 10: 6: 1 Diethyl ether: isopentane :ethanol : l-chloronaphthalene 8:6:2:2 Diethyl ether:isopentane:ethanol:pyridine 12:10:6:1 Diethyl ether: isopentane: triethylamine 5:5:2 Isopentane :methylcyclopentane :methylcyclohexane: rthyl bromide 7:7:4:1 a This glass is particularly stable for an all alcohol system. * If cooled slowly, these glasses can be used. They are unstable and crack easily. c Mixed CLS- and trans-pentene-2 available from Phillips Petroleum Co., Special Products Division, Bartlesville, Oklahoma.
1:LTRAVIOLET ABYORPTIOK SPECTRA OF 0-, m-, AKD p-XITROBEKZOIC ACIDS
Vol. 66
Kagakura and Tanakal in terms of the intramolecular charge-transfer involving an excitation of a bonding electron of the highest occupied energy level of benzene to the vacant energy level of the substituent group. In this note an attempt has been made to investigate the absorption spectra of 0-, m- and pnitrobenzoic acids and to explain them according to the coiicept of intramo'ecular charge-transfer absorption. Experimental The near-ultraviolet absorption spectra of benzoic acid and nitrobenzoic acids were measured in mater on a Beckman spectrophotometer RIodel DT; using I-cm. silica cells a t room temperature (27'). In Fig. 1 are shown the absorption curves of benzoic acid and the nitrobenzoic acids. The results are recorded in Table I, where intensities and positions of the maximum absorption bands of o-, m- and p-nitrobenzoic acids are expressed as the logarithm of the molar extinction coefficients and as e.v. units, respectively. The materials used %'ere all B.D.H. reagents. They were recrystallized repeatedly from ethanol and water until they gave constant melting points in agreement with those listed in the literature.
Theoretical Consideration by the Use of Energy Level Diagrams.-In order to explain the maximum absorption bands of 0-,m- and p-nitrobenzoic acids from the concept of intramolecular charge-transfer involving excitation of a bonding electron from the highest occupied orbital of nitrobenzene to the vacant orbital of the carboxyl group, it is necessary to obtain the molecular energy level diagrams of the nitrobenzene and the substituent carboxyl group. These diagrams are determined 011 the basis of experimental results of the ionization potentia1 and the electronic absorption spectrum. The energies of the highest and lower occupied orbitals of nitrobenzene were determined by Nagakura, e2 aL2 The lowest vacant orbital of nitrobenzene could be estimated by adding the excitation energy to the highest occupied orbital. The vacant energy level of the carboxyl group is estimated a t -4.67 e.v.2 Since the lowest vacant orbital of nitrobenzene, ie., VX and that of the carboxyl group, Le., VCO~H are nearly degenerate, as is evident in Fig. 2, there will be strong interactions between these two levels; as a result the first vacant orbital of nitrobenzoic acid will be depressed considerably. In this paper we shall attempt to explain the ultraviolet absorption spectra of nitrobenzoic acids by considering the interaction of the occupied orbital H,, and the orbital VN of nitrobenzene with the orbital VCO?Hof the carboxyl group. The highest occupied orbital Wl of nitrobenzoic acid obtained from the interaction of Hnl with VCO~H Ievel is given by
+
+
+
1 ,j [N,, V C O ~ H { ( H m- T ~ c o ~ H ) ~
Tt'i
4C12CC01H2B2)' I 2 ]
Department of Chemistry, 1:niz:ersity College of Science and Teclrnology, Calcutta 9,India REcesbad October 8 . 1961
Introduction of a substituent group for hydrogen in a benzene nucleus long has been known to cause shifts in the spectrum of the original benzene derivative to longer wave lengths. The near-ultraviolet absorption spectra of some mono-substituted benzenes con1aining nitro, carbonyl and carboxyl groups as substituents have been interpreted by
1)
while the lowest vacant orbital '472 obtained from the interaction of Vn with VCO~H is given as =
1
2
[VN
+
TiCOnH
+
[(VN
+
BCOZH)~
~ ~ 2 2 ~ C O z H * ~ 2 (} 2 1 )~ ~ ]
where " 1 , V Nand VCO~H are equal to the energies of the highest) occupied and lowest vacant orbitals (1) S. Fagakura and d. Tanaka, J. Chem. Phys., 22, 236 11954). (2) 5. Nagakura, J. Tanaka and 31. Kobayashi, z b d , 24, 311 (1986).
March, 1962
.
NOTES
nitrobenzene we expect the existence of two corresponding occupied orbitals Hal and Hn2. Since VCO~Hcan interact with both the levels H,, and H n 2 , for 0- and m-nitrobenzoic acids we should expect two absorption bands a t longer wave lengths. On substituting the carboxyl group in the paraposition of nitrobenzene, a transition of a bonding ~ to the lowest vacant electron from the H P level orbital Vp can be predicted to show an intense band, while the Hn, level of nitrobenzene remains unaffected (Fig. 2). Simple quantitative considerations were made using equations 1 and 2. The calculated results3 are shown in Table I together with the experimental ones.
I. Benzoic Acid.
I.p . IYih-o.8enzoic Acid. 0.Nitro-8enzoic Acid. m.Nii‘ro-ssnwi? Acid.
t
200
230
260
290
320
TABLE I CALCULATED
I
-351
I,
vo
-=9
-5.70 VP -
-9.24
- 105 -
(nitro6enm)
Fig. 2.--Energy
m-Nitrobenzoic acid
3.50
-“ --
-4.92 -
OBSERVED SPECTRA ACIDS(IN WATER)
o-Xtrobenzoic acid
(mp)
- 43u
AND
OF xITROBESZOIC
Calcd. Obsd. spectra, e.v. spectra, e.v.
Fig. 1.-Xear ultraviolet absorption spectra of nitrobenzoic acids and benzoic acids (solvent, water). -7.7,
563
(‘mnifrobenzoic (Osni/tobenzoic (p, nifro6enrok acid) Acid) Acid)
level diagrams of nitrobenzene and nitrobenzoic acids.
of nitrobenzene and the lowest vacant orbital of the carboxyl group. The C1 and Cz are the coefficients of the atomic orbital of a carbon atom to which the substituent is attached in the molecular orbitals HNI and VN of nitrobenzene, while the CCQHis the coefficient of the atomic orbital of the carbon atom in the molecular orbital VCO~H. The p is the resonance integral of the C-C bond joining the two conjugated systems. The value of p is taken as -3 e.v. The excitation from W1to W z gives rise to the near-ultraviolet absorption spectra of the nitrobenzoic acids. As is well known the degeneracy of the highest occupied orbital of benzene is removed by the perturbation of a substituent and therefore for the
p-Nitrobenzoic acid
4.37 4.72 4.94 5 36 4.80
4.59
log
E
3.72
..
..
4.67 5.79 4.46
3.95 4.38 3.92
Discussion Table I shows that the agreement is not very satisfactory in view of the simplicity of the theoretical treatment. Nevertheless, the observed differences in the spectra of the nitrobenzoic acids are qualitatively explained. It would be interesting to see if the absorption spectra of the nitrobenzoic acids could be explained in terms of the intramolecular charge transfer from benzoic acid to the vacant orbital of the nitro group. Since molecular energy level diagrams of benzoic acid are not known, this calculation cannot be undertaken. Moreover, it is desired that in the more exact theoretical treatment we should consider all possible configurations representing the charge-transfer and mix them. Ortho-substituted benzene usually shows a spectrum resembling that of a meta-substituted benzene, but the behavior of o-nitrobenzoic acid is somewhat different from that of m-nitrobenzoic acid and shows a little broad spectrum with only one band at 268 mp. This difference may be attributed to steric hindrance by the adjacent large nitro and carboxyl groups. This steric hindrance may interfere with the coplanar arrangement of the two substituent groups with the benzene nucleus. The p-nitrobenzoic acid shows only one strong charge transfer band while the transition H p -+ Vp a t 3.5 e.v. is not observed, although this is an allowed in-plane transition. Similar transition in many other para-disubstituted benzenes4 is not observed. The weak shoulders of the nitrobenzoic acids at 335 mp cannot be unequivocally accounted for. It may be suggested from the comparison of the (3) The coefficients of atomic orbitals in the molecular orbitals ot nitrobenzene are determined b y the standard LCAO method with the same parameters used in ref. 2. T o determine the coefficients of atomic orbitals in the molecular orbitals of tho carboxyl group the following parameters were chosen, e.g.: Coulomb intogral ao = a. 4Z p ; a (carbon attached t o oxygen) = a0 0 . l p and p,o = 45p where B is the C-C resonance integral. (4) 3. Nagakura and J. Tanaka, J. Chem. Phys., 24, 1274 (1956).
+
564
Vol. 66
NOTES
intensities that they correspond either to the 270 m,u band of benzoic acid or to the 290 m,d band of nitrobenzene, A ”ienzel, J . Chem. Phus., 22, 1623 (1954).
(5)
RECOIL-FREE yRAY TRANSITION OF Fe6’
1N THE BLOOD COMPOKENT HEMIN BY ULRICHGONSER Atomics International, Division of North American Aviation, Inc., Canoga Pa&, California Received October 6, 1981
Massbauerl has shown that the nuclear recoil of an atom emitting or absorbing a y-ray may be absent when the decaying or absorbing atom is bound ;il a crystal. The recoil energy of a free atom, ER(atom), is given in good approximation by
where E, is the energy of the nuclear transition, m the mass of the nucleus and c the velocity of light. If the emitting atom is bound in a crystal and EE(atom) is comparatively small relative to the Debye energy, AWB, where W e is the limiting frequency of the Debye spectrum, a finite probability exists that the entire crystal takes up the recoil momentum resulting in a negligible energy loss by the y-ray. The fraction of recoil free events is proportional to the familiar DebyeWaller factor.
-7
I
Bwf should be large compared to &(atom) aiid 6 large compared to &{niolecule). The ambient temperature should be lorn compared t o the Debye temperature of the organic crystal and also low enough that the fundamental vibrational modes are not thermally excited. I n such a crystal, the Debye-Waller factor consists of a term which accounts for the Debye frequencies and another term which accountsfor the fundamental vibrational modes of the molecule. If the molecule is very large and the recoil momentum is taken up by the whole molecule, ER(molecu1e) in eq. 1 hecomes very small and can become of the order of the natural line width. In this case a large molecule containing the atom Undergoing the transition does not have to be part of a solid crystal. A Mossbauer effect might be expected even from a large molecule in a liquid. A broadening may occur due to convection and Brownian movement (first-order Doppler eff rct) . An investigation was started to search for the recoil-free y-transition of Fe57 in blood and in blood components. The red cells of blood consist of 32% hemoglobin nith the empirical formula [C738II116602ns?;zo3s~~e]4.,Hydrolysis of hemoglobin cleaves the molecule into two fragments: heme, which contains the Fe-atom, and the protein globin. Hemin is the chloride form of heme.2 The iron is bound to four nitrogen atoms by either primary valences or coordinated links. The nitrogen atoms form a plane. The out-ofplane fundamental frequency probably is different from the in-plane frequency. Therefore, using a single crystal of hemin, a directionally dependent Mbssbauer effect can be expected. The recoil energy for a free iron atom, for a rigid hemin molecule, for a hemoglobin molecule and for one red cell emitting or absorbing a 14.4 Kev. y-ray is according to eq. 1 h
ER(Fe): ER(hernin molecule): ER(hemog1obin molecule): ER(one red cell):
I
-03
-0 2
”- 0 I L’ELOCITY
’ OF S O U R C E
d0 2
03
(cm/~rcl.
Fig. l.-Spectrum of the 14.4 lrev. y-ray, produced by the decay of co57diffused into copper metal and a hemin absorber kept at liquid nitrogen temperature.
The class of materials available for Mijssbaucr experiments can be extended to organic molecules nhich contain the atom undergoing the nuclear transition. In order to calculate the fraction of recoil free events for this case, one has not only \o consider the Debye frequencies of the organic crystal but also the fundamental vibrational frequencies, uf,of the emitting atom in the molecule. To get an appreciable blossbauer effect, (1) R. L. Mbssbauer, Z. Phusik, 151, 124 (1958).
2 X 2 X 2 X 10-14
e.v. e.v. e.v. e.~.
These values have to be compared with the natural line width of the 14.4 kev. y-transition of FeS7v,-hich is r = 5 X 10-9 e.v. The natural line width is smaller than ljJR (hemoglobin) but wide compared to &(red cell). A source of C057 diffused in Cu was used. The absorber consisted of 133 mg. henlin/cm.2 held between thin aluminum foils which were mounted in a holder aiid connected t o a liquid nitrogen cryostat. The measured counts as a function of velocity of the source are shown in Figure 1. If these data are compared to the data for the same source with a stainless steel absorber, it is seen that the hemin curve is shifted toward the center (zero velocity) by a very small amount (about 0.01 cin./sec.) and the line .vvidth is nearly twice as large. The isomer shift (or chemical shift), which is a measure of the electron density at the nucleus, is indicative of the chemical binding of the iron atom. A large isomer shift was observed in Fez+ (2) L. F. Fieser and M. Fieser, “Organic Chemistry,” D. C. Heath & Co., Boston, Mass., 2nd ed.. p. 465.
