NOTES
58 1
Table I Temp., OC. 24.0
21.5
k k k k
(crn.-l), (cm.-l), (cm.-I), (cm.-I),
ref. 1 ref. 2 ref. 3 this work
1
I
1.67
25.0
30.7
30.5
1.2 1.46 1.80
2.09
2.50
1.47
l
1
I
I
l
80.00
45.5
62.5
80.0
2.63
4.55
7.05
the emission a t this wave length from low-pressure mercury lamps, the absorption data listed in Table I were obtained. Our results agree with that a t 25.0" by Barrett and Mansel12 and are considerably lower than those reported by Weeks, et aL3
Phosphorus-31 Chemical Shifts of Quaternary Phosphonium Salts by Samuel 0. Grim, William McFarlane, Edward F. Davidoff, and Tobin J. Marks' Department of Chemistry, University of Maryland, College Park, Maryland (Received July 12, 1966)
182
186
184
188
mP
Figure 1. Absorption of liquid water (in k, crn.-I) a t several temperatures: 0, ref. 1; A, ref. 2; 0, ref. 3; 0, present work.
it was necessary to obtain reliable values for the absorption over a wider range of temperature. Experimental Section Absorption spectra were measured with a Zeiss PMQII far-ultraviolet spectrophotometer, in which the whole optical path was flushed with nitrogen (99.9% pure; 2 1. min.-' flow rate). Absorption spectra of triply distilled water (redistilled from alkaline permanganate and then from phosphoric acid in an allglass still) in Suprasil quartz cells (1.00-mm. path length) were measured against empty cells in a temperature-controlled cell holder ( zkO.5"). Results The absorption spectrum of water a t different temperatures is presented in Figure 1, which includes available literature data. For 184.9 mp, which is particularly important in photochemistry because of
It has been shown that the phosphorus chemical shifts of tertiary and secondary phosphines can be predicted very accurately from empirically determined which have been additive group contributions ,)'a( assigned for phenyl and various alkyl groups2 This type of behavior had been suggested previously for trivalent phosphorus compounds but the predictions were only moderately successful, and similar attempts to predict the phosphorus chemical shift in "quadruply connected" phosphorus compounds, such as phosphine oxides, were unsuccessful. Groenweghe, Maier, and Moedritzer4 observed a regular variation in the P3' chemical shift of a tertiary, secondary, or halophosphine, or a "quadruply connected" organophosphorus compound upon stepwise substitution of the organic groups by other organic groups. In addition, two very recent theoretical treatises5v6 (1) National Science Foundation Undergraduate Research Participant, 1963-1965. (2) S. 0.Grim and W. McFarlane, Nature, in press. (3) J. R. Van Wazer, C. F. Callis, J. N. Shoolery, and R. C. Jones, J . A m . Chem. SOC.,78, 5715 (1956). (4) L. C. D. Groenweghe, L. Maier, and K. Moedritzer, J. Phys. Chem., 66, 901 (1962).
Volume 70, Number 2 February 1966
NOTES
582
have dealt with the problem of calculation of PSI Table 11: Calculated and Observed P31 Chemical chemical shifts. Shifts of Quaternary Phosphonium Salts Since the number of reported phosphorus chemical shifts in quaternary phosphonium salts is very ~ma11,~-~ Cnlcd A, Obsd A, Compound ppm ppm we undertook a study of phosphonium halides to determine the effect of slight differences in a homologous -23.2" [PLPII -22.0 [PhaMePIBr -22.8 -22.7 series of phosphonium salts on the chemical shift and [PhaEtPIBr -26.4 -26.2 also to attempt predictions similar to those for phos[PhaPrPIBr -25.4 -24.1 phines.2 [Ph3BuP]Br -25.4 -24.0 The phosphorus chemical shifts of 49 phosphonium [Pha(i-Pr)P]Br -29.8 -30.9 salts are reported in Tables I and 11. All chemical [Pha(c-Pe)P]Br -28.3 -30.7 [PhzMe2P]Br -23.6 -22.1 shifts are given for solutions in dimethyl sulfoxide un[PhzMe(i-Pr)P]Br -30.6 -30.9 less otherwise indicated. The measurements were [PhzMe(c-Pe)P]Br -29.1 -29.8 made using a Varian DP 60 nmr spectrometer a t 24.3 -34.3 [PhzEt(i-Pr)P]Br -36.2 Mc/sec in 15 X 120 mm tubes with 85% phosphoric [PhzEt(c-Pe)P]Br -32.7 -34.8
Table I: Pal Chemical Shifts of Some Quaternary Triphenylphosphonium Salts
Compoundd
Obsd A, ppm
[Ph,PMe]Br [Ph3PEt]Br [PhsPPr]Br [Ph3PBu ]Br [Ph3PHx]Br [Ph3P(i-Pr)]Br [ PhaP(sec-Bu)]Br [Ph3P(c-Hx)]Br [PhsP(c-Pe)]Br [PhaPCH(CH~CH&lBr [PhaPCH(CH&H,)( CH2CHzCHa)lBr [PhaP(t-Bu)]I [Ph3PC(CH~)ZCHZCH~]I [PhaPC(CHa)C&olI
-22.7" -26.2' -24.1 -24.0 -24.4 -30.9 -30.2 -26.6 -30.7 -30.1 -30.3 -34.7 -36.7 -35.1
[PhsPCHzClICl [PhsPC7H~]Br [PhsPCHzCH=CHzlBr [PhaPCHzC(CH3)=CHz]C1 [PhaPCHzCH=CHPh]Br [PhaPCHzCOaCH~] Br [PhsPCHzCOCH3]Br [Ph3PCH,OH]Cl [PhsPCHzBr]Br [PhsP(CH&Br]Br [Ph3P(CH&Br]Br [Ph3PCHzPh]Br [Ph3PCHPhz]Br [PhaPCH2(m-NOzCsHa)]Br lPhaPCHz(pNOzC6H4)lElr [Ph3P(CHz)aPPh31Brz [PhsPCHpC6H&H2PPha1Br2
-23.8 -22.6 -21.4 -21 .o -22.2 -20.3 -19.4 -17.7 -22.6' -24.0 -24.2 -23.5 -21.4 -24.0 -23.8 -23.2d
U
-22.8d
'
'Lit.7 [PhtPMeII, -20 in MeOH. Lit.' [Ph3PEt]I, -26 in MeOH. Lit.s -24.0 in CHC13. In MeOH. 6Ph = phenyl, Me = methyl, Et = ethyl, Pr = propyl, i-Pr = isopropyl, Hx = hexyl, e-Hx = cyclohexyl, c-Pe = cyclopentyl.
The JOUTWZ~ of Physical Chemistry
[PhzPr(c-Pe)P]Br [PhEt(i-Pr),P]Br [PhPr( i-Pr),P]Br [PhMe(c-Pe)zP]Br [PhEt(~ - P e ) ~ p ] B r [PhPr(c-Pe),P]Br [MeSIBr [MeaEtPIBr [Me(~ - p e ) ~ P ] B r [hIe(i-Pr)3P]Br [BusEtP] Br [BuZlBr
-31.7 -42.2 -41.0 -35.4 -39.1 -37.9 -25.2 -28.8 -41.6 -46.2 -36.6 -35.6
-32.5 -42.2 -39.9 -36.4 -37.3 -35.6 -25.1 -28.0 -40.0 -45.1 -35.5 -33.9
In CHzCl2.
acid as an external reference in a 5-mm o.d., thin-wall tube inserted concentrically into the sample tube through a serum cap. Within the moderately high conc.entration range (1.0 to 0.13 M) needed for detection of the signal, there was no change of the chemical shift of methyltriphenylphosphonium bromide in dimethyl sulfoxide upon concentration changes. Substitution of bromide by chloride and iodide likewise had no effect on the chemical shift of the methyltriphenylphosphonium cation. However, solvent effects were noted in a brief study (see Table 111) with methyltriphenylphosphonium bromide and more extensive solvent studies are in progress. Table I includes the P31 chemical shifts of 31 triphenylphosphonium salts. From inductive effect con(5) H.s. Gutowsky and J. Larmann, J. Am. Chem. Soc., 87, 3815 (1965). (6) J. H.Letoher and J. R. Van Wazer, J. Phgs. Chem., in press. We are grateful to Dr. Van Wazer for sending us his manuscript prior to its publication. (7) D. B. Denney and L. C. Smith, J . Org. Chem., 27, 3404 (1962), footnote 5. (8) J. S. Driscoll, D. W. Grisley, Jr., J. V. Pustinger, J. E. Harris, and C. N. Matthews, {bid., 29, 2427 (1964). (9) K. Moedritzer, L. Maier, and L. C. D. Groenweghe, J . Chem. Eng. Data, 7, 307 (1962).
NOTES
583
~~
Table IV : Group Contributions to Pal Chemical Shift of a Quaternary Phosphonium Compound
Table III: Pal Chemical Shifts of [MePhsP]Br in Various Solvents Chemical shift, Solvent
DMSO CHzClz MeOH GPrOH
HzO
ppm, v8. 85% HsPOd
-22.7 -22.0 -21.5 -21.3 -20.5
siderations, one would expect that in the series [MePh3P]Br, [EtPh3P]Br, [ (i-Pr)Ph3P]Br, and [ (t-Bu)Ph3P]Br, the shielding of the phosphorus would increase and the chemical shift would likewise move upfield. However, the opposite order is observed. One explanation for this behavior which is consistent with the experimental data is hyperconjugation of the ahydrogens and use of the 3d orbitals of phosphorus e
Ph3PCRZH ++PhaP=C&
He
The greater the number of a-hydrogens (three for methyl, two for normal alkyl, one for secondary alkyl, and zero for tertiary alkyl), the more extensive would be the involvement in hyperconjugation and the larger the shielding of the phosphorus nucleus. The chemical shifts of the simple alkyltriphenylphosphonium salts in Table I can be rationalized rather well by this explanation. More mathematically based arguments have been presented by otherse to explain this phenomenon. Table I1 lists the phosphorus chemical shifts of 24 (including six repeated from Table I) phosphonium salts which contain only those groups for which the up values are known.1° Of these 24 salts, the following numbers contain at least one of the indicated groups: methyl, 9; ethyl, 7; propyl, 4; butyl, 3; isopropyl, 6; cyclopentyl, 8; and phenyl, 18. The Pal chemical shift of these phosphonium salts can be predicted by adding the group contribution listed in Table IV for each group in the quaternary compound. For example, the predicted chemical shift of diphenylmethylisopropylphosphonium bromide is 2(-5.5) - 6.3 - 13.4 = -30.7, whereas the observed chemical shift is - 30.9 ppm. If the group contributions from Table IV (with the exception of phenyl) are plotted us. the corresponding up values, a straight line is obtained. See Figure 1. Thus, the values of Table IV can be replaced (except for phenyl) by up values multiplied by the slope of the line in Figure 1 and the P3Ichemical shift in quaternary
Group
Contribution
Ph Me Et
-5.5 -6.3 -10.1 -8.5 -8.5 -13.4 -11.8
Pr Bu i-Pr
c-Pe
phosphonium salts can be predicted by eq 1. ‘n
A (ppm) = -6.3n
- 5.5(4 - n) - 0.26cup n
A (ppm) = -22.0 - 0.8n - 0.26cup
(1)
where n is the number of alkyl groups. The values calculated from eq 1 are listed in Table 11. The position of phenyl in Figure 1 suggests that its ability to shield the phosphorus atom is much greater in the quaternary phosphonium salts than in the tertiary phosphines. This is consistent with the fact that phenyl can act as a .rr-electron donor to the empty 3d orbitals of phosphorus in the phosphonium compounds and as a n*-electron acceptor of the free electron pair of phosphorus in the phosphines.
0
Pb
Pr, BU
up, ppm.
Figure 1. Group contribution to chemical shift of phosphonium salt us. up.
Work is in progress to expand the list of up values since only nine groups have been assigned to date. It should be pointed out, however, that the PSIchemical shift of 1209 tertiary and secondary phosphines (10) up for cyclopentyl is 21 (determined since publication of ref 2).
Volume 70,Number P February 1086
584
NOTES
and quaternary phosphonium salts can be predicted from the values for the nine groups.
Acknowledgment. We thank the National Science Foundation (G-24443) for support of this work, the Advanced Research Projects Agency, Department of Defense, for purchase of nmr equipment, and M. and T. Chemicals, Inc., for gifts of phosphorus chemicals.
chosen as a model compound. This note describes the intermolecular charge-transfer complexes formed in dichloromethane between NIPC and four electron acceptors, 1,3,5-trinitrobenzene (TNB) , picryl chloride (PC), 2,4,7-trinitro-9-fluorenone (TNF), and tetracyanoethylene (TCNE) .
Experimental Section
Materials. NIPC, PC, TNB, TNF, and dichloromethane were obtained from Eastman Organic Chemicals. TCNE was obtained from K & K Laboratories, Inc. The solid compounds were recrystallized once Charge-Transfer Complexes of from ethanol or chlorobenzene before use. Optical Measurements. Absorption spectra in the N-Isopropylcarbazole visible and infrared were measured with an automatic, recording Cary 14 spectrophotometer. The l-cm by James H. Sharp sample cell was housed in a thermostatible cell compartment through which thermostated water was Xerox Corporation, Rochester, New York (Received July 22, 1966) circulated. The temperature of the sample could be maintained to within h0.5' throughout the temperature range from 5 to 60'. Intermolecular charge-transfer complexes between For the particular complex under study, a series aromatic molecules which can act as electron donors of solutions was prepared in which the acceptor conand electron-acceptor molecules such as quinones, centration was invariant (-2 X M ) and halogens, cyano and nitro compounds are well known small compared to the NIPC concentration which and have been studied extensively. These complexes was usually varied from 0.6 to 0.12 M . In the case are generally characterized by an electronic absorpof the NIPCeTCNE complex, the NIPC concentration in the visible or near ultraviolet which is a unique tion was varied from 0.2 to 0.05 M . In each series, property of the complex. blanks consisting of the acceptor alone and NIPC Mullikenz has considered these complexes to be the alone were run. Any slight absorption due to NIPC result of an acid-base reaction between the aromatic or the acceptor in the region of the complex was submolecule (Lewis base) and the electron acceptor sequently subtracted from the charge-transfer ab(Lewis acid). The ground- and excited-state wave sorption. All samples were run against a l-cm cell functions of the complex are treated as a resonance containing dichloromethane in the reference beam. hybrid between a nonbonded state, J/o(D,A) and a Analysis of the Charge-Transfer Absorption Spectra. dative state $D(D+A-). Thus the ground-state b $ ~ An analysis, similar to that by Benesi and Hildebrand5 wave function is given by I+~G = a$o(D,A) and Keefer and AndrewsY6was used to interpret (D+A-), whereas the excited state is represented by the spectra. The formation of a 1 : l molecular com= a*$D(D+A-) - b*J/o(D,A). This interpretaplex may be represented by the following equilibrium tion has been successful in explaining the energetics between the donor-acceptor pair of the spectroscopically observed charge-transfer bands of such complexes. D + A ~ D . A (1 1 In recent work, Taniguchi, et ~ l .have , ~ described the electrical and optical properties of the charge-transfer (1) For recent reviews see: (a) R. S. Mulliken and W. B. Person, Ann. Rev. Phy8. Chem., 13, 107 (1962);(b) G.Briegleb, "Elektronencomplex between poly-N-vinylcarbazole (PVK) and Donator-Acoeptor-Komplexe," Springer-Verlag, Berlin-Gottingentetracyanoquinodimethane (TCNQ). In a later publiHeidelberg, 1961; (e) R. S. Mulliken, J . Chim. Phys., 61,20 (1964); (d) V. P. Parhi. Russ. Chem. Rev., 31,408 (1962). cation, Hoeg14 applied electrostatic imaging techniques (2) (a) R. S. Muuiken, J . Am. Chem. SOC., 74, 811 (1952); (b) R. S. to the study of the photoresponse of PVK and other Mulliken, J . Phya. Chem., 56,801 (1952). He has shown that the photoOrganic (3) A. Taniguohi, S.Kanda, T. Nogaito. S. Kusabayashi, H. Mikawa, and K. Ito, Bull. C h m . SOC.Japan, 37, 1386 (1964). response may be strongly improved by doping with a (4) H. Hoegl, J . Phys. C h . ,69, 755 (1965). variety of electron acceptors. (6) H. A. Benesi and J. H. Hildebrand, J . Am, Chem. Soc., 71, 2703 In order to determine the effect of charge-transfer (1949). interactions on the photoconductive properties of or(6) (a) R. M. Keefer and L. J. Andrews, ibid., 72, 4677 (1950); ganic crystals, N-isopropylcarbazole (NIPC) has been (b) L. J. Andrews and R. M . Keefer, ibcd., 73,462 (1951).
+
~~
The Journal of Physical Chemistry
