coniplementary to the information contained in the peak shape, a combination of the SRM with the numerical methods utilizing peak models could be expected to widen the range o f t h e applicability of both methods. 6) It is possible for signal processing in the case of a twopeak overlap to be done automatically (11).Intervals of the constant signal ratios can be determined by monitoring the slope and the noise level of the signal ratio curve. The values of the characteristic signal ratios can then be obtained by averaging the signal ratio values contained in the appropriate intervals. Once the characteristic signal ratios are determined, Lhe deconvolution of the peak overlap can be easily achieved by means of Equation 9. For more than two components, the process is more involved and less suitable for an automatic execution. A large number of chromatographic detectors qualify for use in the SRM, provided the differences in their dynamic properties can be satisfactorily balanced. An assembly of
dynamically identical detectors offers obvious advantages. Such a system could be created, e.g., from a few photoionization detectors with the source of ionizing radiation separated from the ionization chamber (23).The selectivity of this detector system can be achieved by filtration of the ionizing radiation.
ACKNOWLEDGMENT
I wish to thank Zdenko Sternberg of the Institute Rudjer Boskovi6, Zagreb, Djurdja Deur-Siftar of the OrganskoKemijska Industrija, Zagreb, and Juraj BokiCeviC of the Department of Chemical Technology a t the University of Zagreb, for their valuable assistance in this work. RECEIVEDfor review February 4, 1974. Accepted May 31, 1974. (13) N. Ostojic and 2. Sternerg, Chromatographia, 7, 3 (1974).
Gas-Complex Chromatography: Substituent and Steric Effects R. J. Laub, Vaidhyanathan Ramamurthy,
and R. L. Pecsok
Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822
Specific retention volumes ( Vg), and relative formation constants (K') are found for aromatic hydrocarbons, amines, and a series of P-iony! compounds which are twisted diene systems. Di-n-butyl tetrachlorophthalate (DBTP) is used as the complexing (electron acceptor) stationary phase. Di-nbutyl phthalate (DNBP) and di-isooctyl phthalate (DIOP) were the reference phases. The order of relative formation constants for aromatic hydrocarbons and amines is demonstrate0 to be as expected. The 0-ionyl compounds, however, give anomalous K'values, due to severe out-of-plane deformation effects.
Charge transfer interactions ( 1 ) have been shown by numerous authors (2-7) to enhance GLC separations. However, fundamental solution phenomena may also be investigated with such reagents (8-20). Purnell has demonstrated R. S. Mulliken and W. B. Person, "Molecular Complexes." Wiley-lnterscience, New York. N.Y., 1969. R. 0. C. Norman, Proc. Cbem. Soc., London, 151 (1958). S . H. Langer. C. Zahn, and G. Pantazoplos, J, Chromatogr., 3, 154 (1960). E . Gil-Av. J. Herling, and J. Shabtai, J. Chromatogr., I, 508 (1958). E . Gil-Av and V.Schurig, Anal. Cbem., 43, 2030 (1971). D. V Banthorpe. C. Gatford. and B. R. Hollebone. J. Gas Chromatogr. (J. Cbromarogr. Sci.), 6,61 (1968). 0 . H. Gump, J. Chromatogr. Sci.. 7, 755 (1969). E . Gij-Av and J. Herling, J. Pbys. Cbem., 66, 1208 (1962). M. A. Muhs and F. T. Weiss. J. Amer. Chem. Soc.. 84, 4697 (1962). N. Kotsev and D Shopov. Dokl. Bolg. Akad. Nauk. 21, 889 (1968). ti. Schnecko. Anal. Chem., 40, 1391 (1968). C. Eon. C. Pornrnter, and G. Ciuiochon, J. Phys. Cbem., 75,2632 (1971). J. H Purnell, in "Gas Chromatography-1966,'' A . E. Littlewood, Ed., Adlard. London. 1966, pp 3-20. D. F. Codogen and J. H.Purnell, J. Pbys. Cbem., 73, 3849 (1969). D. L. Meen. F. Morris, and J. H. Purnell, J. Chromatogr. Sci.. 9, 291 (1971) J. H. Purnell and 0. P. Srivastava. Anal. Chem.. 45, 11 11 (1973). 0. E . Martire and P. Riedl, J. Phys. Chem., 72, 3478 (1968). J. P. Sheridan, D.E. Martire, and Y. B. Tewari, J. Amer. Cbem. SOC., 94, 3294 (1972).
that, in fact, charge transfer phenomena should be examined by GLC, rather than UV or NMR spectroscopic techniques (16). A measure of the strength of interaction between charge transfer donor and acceptor species is the formation constant, Kf ( I , 21). For:
D + A *C
where C, A, and D are complex, acceptor, and donor species, respectively. Bracketed terms indicate molar concentrations. Several authors (8-21) have shown that Kf may be obtained from GLC data via the well-known equation:
where KL' is the distribution coefficient for a complexforming solute on an "inert" (noncomplexing) stationary phase, and K L is for the same solute on the phase which contains a small (0-0.2M) concentration of acceptor, [A]. By plotting K L us. [A], Equation 2 allows calculation of K f from the slope and intercept of the line. Formation constants for most species investigated thus far are generally small, on the order of 0.1 to 0.5 l./mole. I t is therefore essential to chromatograph solutes on several columns of varying acceptor concentration, rather than calculate K f from a single determination. Formation constants determined from Equation 2 have been measured by the authors to a lower limit of 0.010 l./mole (22). This can undoubtedly (19) J. P. Sheridan, M. A. Capeless, and D. E. Martire. J.Amer. Cbem. SOC., 94, 3298 (1972). (20) J. P. Sheridan. D.E. Martire. and F. P. Banda, J. Amer. Chem. Soc., 95, 4788 (1973). (21) J. Rose, "Molecular Complexes,'' Pergamon Press, New York. N.Y., 1967. (22) R. J. Laub and R. L. Pecsok, Anal. Chem., 46, 1214 (1974).
A N A L Y T I C A L C H E M I S T R Y , VOL. 46. NO
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be improved with the advent of automated and computerized techniques (23,24). A major disadvantage of this method of Kf determination is that complexing agent solubility is generally low in noncomplexing stationary phases. Only in isolated cases (such as 2,3-dichloro-5,6-dicyanobenzoquinonein di-nbutyl phthalate) has this been overcome. I t can be argued, however, that for those cases where solubility is not a problem, the solvated complexing agent will yield solvent-dependent formation constants. The variation in K f for different “inert” solvents is not severe, however, especially when contrasted to spectroscopic data (16). And, in instances where the solute activity coefficients (or vapor pressures) are known, “relative absolute thermodynamic” formation constants can be calculated. These are said to be solvent-independent (25). An alternative to Equation 2 is the use of pure complexing agent as stationary phase (3, 17). Since the concentration of complexor in itself is large (3-5M), solute-solvent interactions are pronounced. Higher column temperatures can thus be employed. There are several restrictions to this method, however, the most severe being the choice of inert phase. Martire and Riedl (17) have shown that the molar volume and London interactions of reference and complexing liquids must be approximately the same. When this is the case, equilibrium formation constants ( K e J can be calculated:
where VgA and VgD are the specific retention volumes for noncomplexing and complexing solutes, respectively, on the reference phase. VgB and Vgcare for the same solutes on the complexing phase. K’ is the relative formation constant, and is related to the equilibrium thermodynamic constant, Keq,via Equation 4. a A , YA, and A represent the activity, activity coefficient, and concentration of pure complexing agent, respectively. Y A can be estimated by chromatographing several noncomplexing solutes on both complexing and noncomplexing phases (17).Thus, K,, can be calculated from data for two columns, and a knowledge of liquid phase molecular weights and densities. Recently, we reported the determination of vertical ionization potentials (1,d) by GLC (22). Charge transfer formation constants were determined for several butadienes which have known (photoelectron spectroscopy) ZVd values. Kf was then plotted against Zvd to define an empirical curve. Ionization potentials of other butadienes were then found from respective formation constants, and the Kf us. ZVd plot. Steric hindrance to charge transfer interactions was readily apparent. Substitution of bulky alkyl groups (such as isopropyl) on the end of the butadiene skeleton resulted in an unexpected increase in apparent ionization potentials. The order of increasing hindrance was: methyl < ethyl < propyl < isopropyl. An anomalous compound was 2,4-dimethyl-1,3-pentadienein which methyl-methyl repulsion causes rotation about the central single bond (26). The diene system is then partially deconjugated, causing an increase in Zvd (from the approximated value of 8.33 eV to the experimental value of 8.83 eV). T o explore possible out-of-plane deformation effects, a (23)M. Goedert and G. Guiochon, Anal. Chem., 42,962(1970). (24)M. Goedert and G. Guiochon, Anal. Chem., 45, 1188 (1973). (25)C.Eon and B. L. Karger, J. Chromatogr. Sci., 10, 140 (1972). (26) W. F. Forbes, R . Shilton, and A. Balasubramanian, J. Org. Chern., 29, 3527 (1964). 1660
series of P-ionyl compounds (27) were synthesized for this research. These are diene systems which are highly hindered sterically. Twisting of the conjugated system around the central single bond thus occurs, which can be measured by NMR-spectroscopic decoupling experiments (28, 29). Because of the high boiling points (ca. 200 “C) of most of these compounds, it was necessary to use a column temperature of 100 “C. Charge transfer interactions become small a t elevated temperatures, however, and so the pure complexing agent, di-butyl tetrachlorophthalate (DBTP) was used as the stationary phase. We chose to examine d-nbutyl phthalate (DNBP) and di-isooctyl phthalate (DIOP) as reference phases, which do not fulfill the criteria of “suitable” inert solvents, as discussed above, since they do not have the same molar volumes and polarizabilities of DBTP (indicated by very different distribution coefficients of noncomplexing solutes on each of the phases). Therefore, to ensure that charge transfer forces were indeed being examined (as opposed to other solution effects), several well-characterized aromatic hydrocarbons were also run. Relative formation constants were then calculated from Equation 3.
EXPERIMENTAL Apparatus. A Hewlett-Packard Model 402 dual flame ionization gas chromatograph with a Varian Model A-25 recorder were employed. Two-foot by 2.8-mm i.d. silanized (HMDS) glass U-columns were used. Reagents. Supelcoport (80/100 mesh) was the solid support. T h e stationary phases were obtained from Applied Sciences Laboratories, Chemical Research Services, and Supelco. T h e preparation of cis- a n d trans-P-ionyl compounds have previously been described (27). Those used in this study are listed below; each was purified by molecular distillation for the NMR experiments.
trans
cis
R . R = L -CH, 2. -CHIOICCH 3. - C H t C i l )OIC‘CH, 1.-CO,CH
Procedure. T h e GC column temperature was maintained a t 100 f 0.1 OC. Helium a t 40 ml/min was the carrier gas. Other experimental details have been described elsewhere (22, 28, 29).
RESULTS AND DISCUSSION Aromatic Hydrocarbons. If Equation 3 is a reflection of charge transfer interactions, then K’ values will increase as the ionization potential of donors decreases. T o test this, several aromatic hydrocarbons which have previously been characterized (22) were used. Specific retention volumes ( V g )and ratios of V , to Vg,r6 ( V gfor cyclohexane, the internal standard) on DNBP were extrapolated to 100 “ C and are given in Table I. Langer’s data a t 100 “C ( 3 ) were used to calculate V , and V,/V,,c6 for the same compounds on DBTP. K‘ values were then calculated from these data, and are also listed in Table I, along with vertical ionization potentials of the aromatic hydrocarbons (22). Several important facts emerge from Table I. The relative formation constants fall in the predicted order: the lower the ionization potential, the larger the charge trans(27)V. Ramamurthy, Y. Butt, C. Yang, P. Yang. and R. S. H. Liu, J. Org. Chem., 38, 1247 (1973). (28)B. Honig, B. Hudson, S. D. Sykes, and M. Karplus, Proc. Nat. Acad. Sci. U.S., 68, 1289 (1971). (29) V. Ramamurthy, T. T. Bopp. and R . S. H . Liu. Tetrahedron Lett.. 3915 ( 1972).
A N A L Y T I C A L C H E M I S T R Y . VOL. 46, NO. 12. OCTOBER 1974
Table I. Ionization Potentials (22), Specific Retention Volumes, and Relative Formation Constants for Aromatic Hydrocarbons with DNBP and DBTP at 100°C DBTP
DNB P
vE, r n l l
CornRd
Cyclohexane ( r e f e r e n c e) Benzene Toluene o -Xylene m-Xylene p -Xylene
K'
g
...
33.36 9.25 8.82 8.57 8.57 8.46
63.36 131.7 371.8 274.9 263.5
2.078 4.320 12.19 9.020 8.641
2.072 4.764 13.52 10.25 10.63
69.10 158.9 451.1 341.8 354.4
- 0.003
0.013 0.110 0.136 0.230
Table 11. Ionization Potentials (33, 34) and Relative Formation Constants for Substituted Aromatic Amines Comud
S t u c t ure
tVd,e V
''cis
7.90
33.02
27.18
7.75
35.16
28.93
7.75
37.34
30.73
7.65
40.10
33.00
...
40.44
33.28
...
16.54
13.61
'trans
A. Anilines
1. Aniline 2. o -Toluidine
QKH. \
p".
3. n7 -Toluidine
CH
fH
4.
p -Toluidine
H C *NH.
5. 2,4 -Xylidine ("
B . N, N - D i m e t h v l a o i l i n e s
1. ,V,S-Dimethylaniline
@NiCH
1
WCH
2. A\T,S-Dimethyl-o-toluidine
7.37
3.685
3.033
3. N,,V-Dimethyl-wz-toluidine
7.35
...
...
4. lV,N-Dimethyl -i,-toluidine
7.33
18.86
15.52
7.25
..
7.22
2.764
2.275
7.17
3.540
2.913
)
CH
H
,e CP
5. ,V,,V -Dimethyl -3,5 -xylidine H
6. ,\~,A\r-Dimethyl -2,6 -xylidine
...
,c' 4
X
C
H
1
CH
7. ,\r.,V-Dimethyl-2,4 -xylidine c?iCHB)
CH
C . Increased N S u b s t i t u t i o n
1. Aniline
2. ,V -Me thy laniline
@KHCH,)
3. 1V.N-Dimethylaniline
&-N(CH
I.
7.90
33.02
27.18
7.60
26.52
21.83
...
16.54
13.61
A N A L Y T I C A L C H E M I S T R Y , VOL. 4 6 , NO. 12. OCTOBER 1 9 7 4
1661
_
_
_
~
_-
Table 111. Approximate Out-of-Plane Deformation Angles and Relative Formation Constants for 3-1onyl Compounds A n ~e ,l
Cumpd
Mean,
trins
K'ClS
A. I i . ( l , / 4
R =1 2. 3. 4
26-34 28-36 28-36 26-34
30 32 32 30
1.232 1.316 1.247 1.191
1.283 1.373 1.300 1.242
B. c i s R' = 1. 2. 3. 4.
30-38 32-41 37-50 33-45
34 37 44 39
1.231 1.293 1.267 1.246
1.284 1.348 1.321 1.299
fer interaction (K'). Langer ( 3 ) ,in fact, made use of this phenomenon to separate m- and p-xylene on di-n-propyl tetrachlorophthalate. Compounds with ionization potential greater than 9 eV (illustrated by benzene) give weak charge transfer interactions which are on the order of experimental error, In addition, 0 - and m-xylene are not in complete agreement. However, if 0.123 is the correct K' value for both (the mean of the two), the error is only 0.013.Finally, the magnitude of the K' values is probably not of great significance. DNBP has an appreciably different molar volume and likely gives different London interactions than DBTP. Therefore, the relative order and not the magnitude of the K' values is important. These values, which indicate relative charge transfer interactions, increase with decreasing ionization potential, as expected. K' values calculated from Equation 3 thus appear to offer a means of examining charge transfer in other systems. Substituted Aromatic Amines. T o verify that K' is directly related to charge transfer interactions (hence, inversely to Ivd),we have examined the data of Cooper, Crowne, and Farrell (30, 31) who chromatographed several substituted aromatic amines on 2,4,7-trinitrofluorenone (TNF) and silicone oil. We have calculated K' values from their data ilia Equation 3;these are listed in Table 11. Since two internal standards (cis- and trans-decalin) were used, we list both K'cis and Krtransvalues. Also given in Table I1 are vertical ionization potentials we have taken from the literature. K' values for the amines in Table IIA follow the expected trend: lower ionization potentials yield larger relative formation constants. However, the data in Table IIB are erratic. .V,N-Dimethylaniline (No. 1) and N,N-dimethyl-ptoluidine (No. 4) fall in the expected order. The former compound has a higher ionization potential and so yields a smaller K' value. However, No. 2, 6,and 7 show no discernible trends, and yield K' values which are appreciably smaller than No. 1 and 4. Substitution in the ortho position is known to cause out-of-plane deformations for N,N-dimethylanilines (32). Steric hindrance to charge transfer may therefore be responsible for the anomalous behavior of these compounds. Table IIC demonstrates the effect of in-
creasing N-substitution a t the nitrogen. Apparently, donoracceptor mutual approachability is hindered by the Nmethyl group(s). Thus, even though Ivddecreases, so does
K'. The data in Table I1 illustrate the very large effects that steric hindrance can cause in charge transfer complexation. When problems of this nature are investigated, therefore, it is crucial to use systems in which steric effects are either negated, or can be determined. For separation purposes, however, these effects can be very advantageous. For example, the relative volatility ( a )of N,N-dimethyl-2,4-xylidine and N,N-dimethyl-2,6-xylidine is 1.13 on silicone oil. However, (Y = 1.45 on TNF, and the order of elution is reversed. @-Ionones. The approximate out-of-plane angles for these compounds (which correspond to twisting around the central single bond) are given in Table 111. The relative formation constants with cis- and trans-decalin (Pcisand Wtrans)obtained on DBTP (DIOP reference) are also presented. Angles for the trans compounds are very nearly the same. Hindrance in these systems is due to hydrogenmethyl repulsion in all four cases, and therefore the angles are.equa1, regardless of R. This is not the case for the cis compounds, however. As the size of R' increases, the angle becomes larger. An examination of K' values seems to indicate increased charge transfer for the trans compounds. As the alkyl group changes, the interaction becomes larger in the order: R = 4 < 1 < 3 < 2. Steric effects can be ruled out as the cause, since K' would then be expected to decrease with increasing size of R. However, K' values for the cis compounds increase in approximately the same order as the trans materials. This is not what was expected. As the angle increases, K' values should decrease, when in fact they are erratic. Apparently, charge transfer is not involved (or a t least is masked) and other solution effects are responsible for the order in K' values. Work is now in progress on compounds where the out-of-plane deformation angles lie within Oo to about 10-15', the apparent upper limit of the GLC method. When out-of-plane twisting or other steric effects are not pronounced, K' values appear to offer a valid measure of charge transfer interactions. Martire and coworkers (35) have recently compared K,, values calculated from Equation 4 to Kf values found uia Equation 2. The agreement was excellent for Y D = 0.993 f 0.002.Thus, K', Kf, and K,, values found by GLC all offer an internally consistent method of examining charge transfer phenomena. Furthermore, as Purnell has pointed out ( 1 6 ) , the GLC data certainly appear to be a more valid measure of these interactions than previously-used spectroscopic techniques. I t should be noted, however, that much conflicting data have been reported in the literature regarding so-called "charge transfer" interactions. The authors only speculate, therefore, that true charge transfer is operative in the systems reported here, since for example, polarizability and van der Waals interactions may offer an equally plausible explanation for our results. Clearly, much work remains to be done in the elucidation of solution phenomena regarding these effects.
RECEIVEDfor review April 18, 1974. Accepted June 21, 1974. (30) A . R Cooper, C. W. P. Crowne, and P. G. Farrell, Trans. Faraday Soc., 62, 2725 (1966). (31) A . R. Cooper, C. W. P. Crowne, and P. G. Farrell. Trans. faraday SOC., 63, 447 (1967). (32) E G. McRae and L. Goodman, J. Chem. Phys., 29, 334 (1958).
1662
(33) G. Briegleb and J. Czekalla. Z.Nektrochem.. 63, 6 (1959). (34) P. G. Farrell and J. Newton, J. Phys. Chem., 6 9 , 3506 (1965). (35) H. Liao, D. E. Martire, and J. P. Sheridan, Anal. Chem., 45, 2087 ( 1973).
A N A L Y T I C A L C H E M I S T R Y . VOL. 46. NO. 12. OCTOBER 1974
