March, 11962
NOTES
561
inaccurate. Virtually all owners of these instru- 0.57 f 0.03 c./sec.' However, for numerical ments recognize this problem, and most have solved evaluatim of resolution, line widths a t half-height it by installatiou of a precision frequency ~ o u n t e r . ~are refera able,^ With this addition these n.m.r. instruments truly Experimental become ,spectrometers, reprodixibilit'y of measureThe n.m.r. equipment arid techniques haye been dement between different laboratories being 50.001 scribed in detail.3 The p-anisaldehyde, from the Eastman p.p.m., or one part in lo4 of the peak separation Kodak Co., was distilled before use to removc oxidation products; a 4.OY0 (by volume) solution of it was made in studied? "spectro grade" carbon tetrachloride (hfat,heson Coleman The recent introduction of the remarkablg Varian & Bell) which already contained 1.0% (by volume) of A-60 spectrometer, which combines convenience of tetramethylsilane (Anderson Laboratorirs, Divieion of the Stauffer Chemical Co.). Sixteen n.m.r. tubes were filled operation with very high reso1utionj6has prompted with this solution; each tube then was sealed after having a reconsiderat'ion of the question of spectrometer benn purged with a fine stream of oxygen-free nitrogen to accuracy, since the calibration of this instrument is sweep out dissolved oxygen. Sample tubes so prepared have dependent on the accura.cy of certain potentmiom- remained unchanged for a period of two years. Caution: n.m.r. t,ubes should no2 be exposed to a high level of eters. There is no method for internal calibra- These over a long period of time. One of the tubes, in tion, though p-rouision has been made for intro- illumination bright light for several weeks, mas found by n.m.r. analysis duction of sidebands if one should have an audio- to contain some chloroform and p-anisyl chloride, formed by oscillator (which of course must be monitored by a the free-radical reaction frequency counter). It seems likely that most hv Ar-CHO + CCl, --+Ar-COG1 C".% owners will not provide this a,ccessory equipment. Inasmuch as t'he manufacturer does not claim iniA very large number of n.ni.r. measurements were made, tial accuracy better than AQ.102 p.p.m., there is a 12 separate determinations being made on each of the 16 reasonable chance that, with use, these instruments tubes. It thus was possible to reduce the standard deviation the averaged value to zkO.0005 p.p.m. even though the may develop yet more serious errors in spectral in standard deviation for a single determination, namely position measurement despite excellent resolution 1-0.007 p.p.m., was much higher. The fundamental acand general performance. Accordingly it should curacy of this procedure is indicated by the remarkably be regarded as rather importmtt to check, occasion- good agreement, wit,hin +0.001 p.p.m., of odr averaged value a t 25.0°, 0.1970 T, -I: 0.0005, with final results from ally, the calibration of all A-60 spectrometers. each of several other careful workers6; in most cases their For this purpose we recommend the formyl peak standard deviations for a single measurement were appreciof p-anisaldehyde, which is a t the extreme low-field ably smaller than ours. series of 26 measurements was made a t +77", end of the normal proton spectrum, +0.197!, 7 theA further Varian variable-temperature apparatus and probe (i~0.0005). The large sepa'ration, nearly ten modification being employed, and the formyl peak was p.p.m., between this peak and that of the internal found to have shifted very slightly, to +0.181 T, ztO.003. reference, tetramethylsilane, is desirable from the From this observation the temperature coefficient of the st,andpoi:nt of magnifying percentage errors; how- 7-value for the formyl peak (assumed constant over the temperature range studied)was evaluated as -0.0003 p.p.m. ever t'hiEi is also a shortcoming in that the usual per degree temperature rise. This correction was applied to spectrum produced by the A-6'0 spectrometer does certain results obtained5 a t slightly difierent temperatures. not include peaks below +1.67 T , making it neces(7) First observed by C. A. Reilly at the Shell Development Co. sary to rise the "spectrum-offset" device to bring the form;yl peak on scale. The lorn 7-value is not a serious limitation, however, since this offset feature is required whenever a region of the spectrum is to SOLVEST GLASSES FOR LOW be expaiilded for more detailed investigation; thereTEMPERATURE SPECTROSCOPIC fore this unit also should be checked for accuracy. STUDIES The methoxyl peak, a t 6.139 5 0.002 7,may be used as a test of linearity, and the aromatic peaks BYDONALD R. SCOTT^ AXD JEAN B. ALLISON' provide a convenient visual est'irnateof performance. Spectroscopy Laboratory, Chemastry Department, Universitg of Houston, Houston 4, Texas A better .test of resolution is seen in t'he formyl peak Received September BS. 1D61 itself, which is a closely-spaced triplet having J = We have compiled in the accompanying Table I a (2) 1. T. Arnold and 1LI. T. Packard. J . Chem. Phys.. 19, 1608 (1951). series of solvent systems which are particularly (3) G. V. D. Tiers, .I. Phys. Chem., 62, 1151 (1958). useful for electronic emission spectral studies a t (4) For example, the 522-B counter, manufactured bv the Ilewlettliquid nitrogen temperature, 77°K. All of these Packard Co., Palo .41to, Calif.: any counter giving precision t o f0.1 systems form transparent, rigid glasses at that c./sec. is equivalent. ( 5 ) A cooperative study was carried out for the N M R Subcommittemperature when proper precautions are taken to tee of A.S.T.RI. Committee E-I3 on P,bsoi.ption Spectroscopy b3' the ensure that all components are anhydrous and of authors togeOher with the following (amon,%others). At 40 Rlo./sec.: high purity.2 Many of these glasses have been P. C. Lauterbur, Mellon Institute; C. A. Reilly, Shell Development employed in this Laboratory and in others for C o . ; C. R I . Huggins, General Electrio Research Lab.; R. E. Glick, Pennsylvania State Univ.; D. P. Ames, bfonsanto Chemical Co. several years for low temperature electronic specAt 60 Mc./eec.: A. A. Bothner-By and B. L. Shapiro, Mellon Intral studies. However, several of the systems are stitute: .I. N. Shoolery and L. F. Johnson, Varian Associates; E. D. known to be original with this Laboratory. Several Becker. A'atl Inst. of Health; K. J. Palmer, U.S.D.A. Western Util. of the more unusual systems are useful when one Research Div.; N. F. Chamberlain, Humble Oil Co.: M. T. Rogers, Michigan State Univ. All of the above workers reported final rewishes to study a compound which has a limited siilts for the s-value of the formyl peak o l panisaldehyde f a t 4.0% solubility in the usual hydrocarbon or hydroxylic concn. by volume in CC1a) within f0.001 p.p.m. of the "best averaged
+
value" obtained in our measurements. (6) G. V. I). Tiers, J. Phys. Chem., 66, 1916 (1961).
(1) Robert A. Welch Foundation Pre-doctoral Fellow. ( 2 ) W. J. Potts, Jr., J . Chem. Phys., 21, 191 (1953).
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).
