157
J. Phys. Chem. 1992,96, 157-164
Bond Polarity Index: Application to Group Electronegativity Lynne H. Reed and Leland C. Allen* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (Received: June 21, 1991; In Final Form: August 26, 1991)
In spite of its potential usefulness to synthetic and structural chemists, there has been no consensus on a quantitative group electronegativity scale nor a manifestation of confidence in existing definitions. The present effort seeb to resolve this problem by (1) employing the bond polarity index, BPIAB, a simple, well-defined quantum mechanical measure of the one-electron energy difference between adjacent atoms A and B in a molecule and (2) demonstrating an excellent correlation between BPIAB and experimental results from electron spectroscopy for chemical analysis (ESCA) core-electron binding energies, the most readily interpretable measurements of substituent electron withdrawal. Correlation with substituent constants (F and up),two other relevant experimental measures, is also good. Correlations with "C NMR chemical shifts and IJ, coupling constants, previously proposed as measures of group electronegativity, have been investigated and comparisons with other definitions of group electronegativity are included.
Introduction
Calculation of BPI
Inorganic and organic synthetic and structural chemists have long acknowledged the desirability of a quantitatively useful group electronegativity scale and a variety of schemes are currently available. These include the classic work of Wells utilizing bond vibrational data,' the empirical covalent boundary potentials of Inamoto and Masuda,2 the use of Mulliken population analysis by Marriott, Reynolds, Taft, and Topsom? Mullay's bond orbital electronegativity formulation,4 application of topological electron partitioning by Boyd and E d g e ~ m b eand , ~ Bratsch's definition using the harmonic mean of Pauling's atomic scalea6 Each of these yield valuable insights into the nature of substituent inductive effects. Eowever, for some time there has been a tendency among practicing chemists to regard group electronegativity as an old fashion and unpromising concept. With this in mind, we have investigated the possible application to group electronegativity of the recently devised energy index, EIA, for a specified atom in a molecule and its associated bond polarity index, BPIAB, which measures the ionicity of bond AB.' These indices also have application in the characterization of rotation and inversion barriers, the anomeric effect, and bonding in solid-state chemistry. An encouraging new aspect of the BPIAB approach is its strong correlation with the most direct experimental data on bond polarity (ESCA core-electron binding energies). EIA is the expectation value of a quantum-mechanical operator which defines the average one-electron energy of valence electrons for an atom in a molecule, and BPIAB is the average one-electron energy difference between two adjacent atoms, A and B, when the homonuclear covalent contribution is subtracted out. BPIAB was shown (Tables I and I1 of ref 7) by comparisons with computed atomic charge distributions and dipole moments to yield a chemically useful definition of bond polarity. Here we use BPI to defme a scale of group electronegativities by considering a series of molecules RX where R is a given reference (such as CH3) and X is the group for which electronegativity is desired. Calculation of BPIRx, the BPI between the attaching atom of R and the attaching atom of group X, gives the polarity of the R-X bond, and since bond polarity is equated to electronegativity difference, BPIRx gives a measure of the electronegativity of the X groups relative to the reference R. Since it is a molecular parameter, it takes into account not only the electronegativity of the attaching atom but also effects from hybridization changes and the electronegativities of the other atoms in the group. Results for 25 organic and inorganic groups using four different references are given below. A comparison is made between the differently referenced series, with experimental data and with other group electronegativity schemes.
BPIAB as given in ref 7 is defined as the covalent-referenced difference in the energy of an average valence electron in atom A and one in atom B:
*To whom all correspondence should be addressed.
BPIAB = (EIA - EIAre') - (EIB- EIBre') where EIA, the energy index of atom A? is the average oneelectron energy of the valence electrons of atom A in a molecule: N
EIA =
1
5:
WA) M N M(A) M FCij*Ci$jk/Eni ECij*C,$j, I J k
Cnis
C
Covalent referencing refers to the need to subtract out the homonuclear, pure covalent bonding component in order to obtain bond polarity from the difference EIA - EIB. For example, to calculate BPIcN in the molecule H3C-NH2, the EIc and EIN are obtained from the molecule itself, EIcrCfis obtained from the homonuclear reference molecule H3C-CH3, and EINnfis obtained from H2N-NH2. Calculations were performed with a b initio MO-SCF wave functions obtained from GAUSSIAN 86'' in the R H F approximation (UHF for radicals) using the 6-31G* basis set.I3 All geometries were 6-3 1G*-optimized structures14 to ensure consistency and to (1) Wells, P. R. Prog. Phys. Org. Chem. 1968, 6, 111. (2) Inamoto, N.; Masuda, S. Chem. Letr. 1982, 1003, 1007. (3) Marriott, S.; Reynolds, W. F.; Taft, R. W.; Topsom, R. D. J . Org. Chem. 1984, 49, 959. (4) Mullay, J. J . Am. Chem. Soc. 1985, 107, 7271. (5) Boyd, R.; Edgmmbe, K. E. J . Am. Chem. Soc. 1988, 110, 4182. (6) Bratsch, S. G. J . Chem. Educ. 1985,62, 101. (7) Allen, L. C.; Egolf, D. A,; Knight, E. T.; Liang, C. J . Phys. Chem. 1990, 94, 5602. (8) For infinitely separated free atoms, EIAis just the occupation weighted average of the one-electron atomic orbital energies for the valence electrons of ground-state free atoms and this quantity has previously been defined as hpoE9 Reference 9 shows that the values of x, determined from the National Bureau of Standards high-resolution atomic energy level data closely matches the widely used Pauling'" and Allred and Rochow" electronegativity scales. Practicing chemists have employed these two atomic electronegativity scales almost exclusively for the last 59 years, and therefore agreement of EIAwith these numbers provides a strong argument for its validity. (9) Allen, L. C. J. Am. Chem. Soc. 1989, 111, 9003. (10) Pauling, L. J . Am. Chem. SOC.1932,54, 3570. Updated by: Allred, A. L. J. Inorg. Nucl. Chem. 1961, 17, 215. (11) Allred, A. L.; Rochow, E. G. J . Inorg. Nucl. Chem. 1958, 5, 264. (12) Binkley, J. S.; Frisch, M. J.; Raghavachari, K.;Defrees, D. J.; Schlegel, H. B.; Whiteside, R. A.; Fluder, E. M.; Seeger, R.;Fox, D. J.; Head-Gordon, M.; Topiol, S. Gaussian 86 release C; Carnegie-Mellon University: Pittsburgh, PA, 1987. (13) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (14) Optimizations performed by us (results available from the authors by request) or obtained from: Whiteside, R. A,, Frisch, M. J., Pople, J. A., Ed. The Carnegie-Mellon Quantum Chemistry Archive; Department of Chemistry, Carnegie-Mellon University: Pittsburgh, PA, 1983.
0022-365419212096-157%03.00/0 0 1992 American Chemical Society
158 The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
Reed and Allen
TABLE I: Bond Polarities“ for X-CH?, X-NH,, X-OH, and X-H Molecules and Energy Indexes’ for Radicals X X BPIyp B P I ~ N BPIvn -EIY BPIvu
BeH BH2 SiH, H CHZCH, CH3 CH2NH2 PH2 CH2OH CHCHZ CH2F COCH, CONH2 NH2 CCH NO SH CHO COOH OH C1 CF3 NO2 CN F
-0.1366 -0.0401 -0.0351 -0.0315 -0.0035 0.0000 0.0044 0.0121 0.0239 0.0273 0.0410 0.0442 0.0478 0.0478 0.0498 0.0581 0.0648 0.0650 0.0837 0.0950 0.1206 0.1269 0.1523 0.1527 0.1893
-0.1758 -0.0324 -0.0626 -0.1552 -0.0512 -0.0478 -0.0434 -0.0132 -0.0159 -0.0044 0.0045 0.0387 0.0318
0.0000 0.0221 0.0544 0.0292 -0.0022 0.0604 0.0477 0.0816 0.0842 0.1256 0.1317 0.1474
-0.2511 -0.1 110 -0.1046 -0.2821 -0.0947 -0.0950 -0.0885 -0.0535 -0.0635 -0.0500 -0.0452 -0.0212 -0.0215 -0.0477 -0.0269 -0.0186 -0.0141 0.0008 0.0031 0.0000 0.0339 0.0354 0.0832 0.0867 0.1015
0.4086 0.5264 0.5177 0.4980 0.6253 0.6238 0.6360 0.5448 0.6667 0.6291 0.7045 0.7290 0.7499 0.6993 0.6403 0.8108 0.5948 0.7424 0.7912 0.7978 0.6680 0.8908 0.9935 0.7164 0.9570
-0.1902 -0.0596 -0.0830 0.0000 0.0241 0.0315 0.0223 -0.0171 0.2514 0.0733 0.0772 0.0610 0.0715 0.1552 0.1209 0.1358 0.0749 0.0906 0.1115 0.2821 0.1750 0.1719 0.4474 0.2398 0.4537
aAtomic units. All values obtained from 6-31G*//6-31G* wave functions. TABLE 11: Bond Polarity Index Compared to Difference in Free-Atom Electronegativitiesfor H,,A-CH3Molecules
X
BPIAc’
BeH BH2 CH, NH2 OH
-0.1366 -0.0401
Axspcb -0.968 -0.493
0.0000
0.000
0.0478 0.0950
0.522 1.066
BPIAC”
F SiH, PH2 SH CI
0.1893 -0.0351 0.0121 0.0648 0.1206
AXspcb
1.649 -0.628 -0.291 0.045 0.325
‘Atomic units. All values obtained from 6-31G*//6-31G* wave functions. Pauling units. Reference 9. allow calculations on molecules for which experimental structures are not available. The species chosen for the present study are 25 groups, each attached to references CH,, NH2, OH, and H. Our notation specifies that the reference be on the right, i.e. X-CH,, giving BPIxc (or BPIxN, BPIxo, BPIxH) and denoting the BPI of the bond between the attaching atom of X and C (or N, 0,H). Since the sign of BPI is indicative of the direction of polarity (a negative BPI indicates a polarity of X less than C), this scheme produces a BPI scale where a small BPI corresponds to low electronegativity and a large BPI to high electronegativity. Calculation of E1 for the radicals (Le. the groups X alone) is included for comparison. BPIxc for X-CH3 Series A series of groups composed of row 1 and 2 atoms were chosen and considered as singly bonded to a CH, reference. Bond polarities are given in Table I in ascending order thereby providing a scale of group electronegativities relative to CH,. Overall, the BPI’s exhibit qualitatively correct chemical trends. Groups such as BeH, BH,, and SiH3 are found to be less electronegative than CH,, while the halogens, CN, NO2,and OH are found to be a good deal more electronegative. Replacement of H on the CH, by a more electronegative substituent, such as NH,, OH, or F, produces an increase in BPIcc. Likewise, there is the expected increase in bond polarity through the series H3CH2C-CH,, H2CHC-CH3, HCC-CH3 . BPIAcversus AxACfor the H,,A-CH3 Subset. By considering a subset of the groups, those composed of a single heavy atom (A) surrounded by the appropriate number of hydrogen atoms, we can compare the calculated BPI’s to the differences between atomic electronegativities AxAC. These BPI’s are listed in Table
f A = row 1 atom
-1.0
-0.5 0.0
0.5
1.0
1.5
2.0
A ~ A C(Pauting units) Figure 1. CH,-referenced BPI (BPIxc) versus atomic electronegativity difference ( A x A C ) for AH, groups. BPIAc is shown to provide a qualitatively correct measure of bond polarity.
I1 along with AxAc, here ordered by atomic number of A, and BPIAc is plotted versus AXAC in Figure 1. Within each row, BPIAc and AxAc both show an increase with increasing atomic number, i.e. as we go across a row in the periodic table. Thus BPI is shown to be providing a qualitatively correct measure of bond polarity for these bonds. This is also a good illustration of the limitations in using atomic Ax and the need for the in situ measure. Thus a single value of Ax can correspond to more than one BPI value, e.g. Ax does not differentiate between X = CH2CH3and X = CH2NH2. EIx vemw BPIXp The energy index (EI) of the attaching atom of each X group obtained from calculations on the X radicals is given in Table I. It had been believed previously7 that the intuitively appealing definition of group electronegativity as the E1 of the open-shell attaching atom perturbed by its ligands would parallel the results obtained using CH3 as a reference. It was reasoned that a t infinite internuclear separations homolytic cleavage of the C-X bond would formally eliminate the need for the homonuclear covalent reference. The results calculated here show that the EIx of the X radicals do not in fact agree with BPIxc. (The seeming parallelism noted previously7is an artifact of plotting the data in order of E1 rather than BPI.) Thus the fluctuations in the Elx curve of Figure 2a, which plots X groups in ascending order of BPI, show that the perturbed atom approach gives a much different ordering of groups than BPIxc. A least-squares line fit through each group of points, Figure 2b, displays a similar upward trend in both cases, but the lines are not parallel. (Likewise, as shown later, EIx does not provide as good a correlation with experimental data as BPIxc.) X-NH2 and X-OH Series
Since the electronegativity of a group would be expected to be dependent on the particular reference to which it is bonded, BPI’s were calculated using NH2 and O H as references instead of CH, (Table I). Since the electronegativities of the attaching atom for these references (xsp = 2.544, 3.066, and 3.610 for C, N, and 0, respectively) span a good part of the whole range of electronegativity values, we expect this to be a rigorous test of the internal consistency of the BPI values obtained. And indeed, we see a remarkable consistency in BPI’s relative to the three references. Figures 3a-c plot BPIxc versus BPIxN, BPIxc versus BPIxo, and BPIxNversus BPIxo, respectively, and in each case a least-squares line is fit to the data and the correlation coefficient ( R ) is given. A high degree of correlation between the three series is apparent and, noting that the X = H point (discussed further below) is primarily responsible for the slightly lower BPIxc BPIxN and BPIxc BPIxo correlations, refitting the data in Figure 3a,b without X = H produces an even higher correlation ( R = 0.972 and 0.986, respectively). By comparison, correlation between EIx and BPIxN or BPIxo (or BPIxc) is significantly lower (Figure 4). Plots of BPIxc, BPIxN, and BPIxo versus groups (in ascending order of BPIxc) brings out specific groups which are predicted to be in a different order than that in the X-CH, series (Figure sa). The X-NH, and X-OH series show an overall smooth upward trend relative to X-CH, with some important exceptions. The largest deviation is X = H in both BPIxN and BPIxo (see
-
-
The Journal of Physical Chemistry, Vol. 96, NO. 1, 1992 159
Application of BPI to Group Electronegativity
y
I
.3.8051e2 + 1 . 0 4 0 2 ~R
0.950
. I
J'
-0.3
(4
-0.2
-0.1
0.0
1.07
0.2
BPI XC (a.u.)
Y
--
0.1
= - 9.5459e-2 + 1.I569x R = 0.923
BPI XC: y .5.0846e-2 + 8.3583e-3x .El X: y 0.49703 + 1.5622e2X A
0.81
sa
m
0.2 1
(b)
BPI XC (a.u.)
-
0.0
y = -5.3160e-2+1.1164~ R-0.976
/
Figure 2. CH,-referenced BPI (BPIxc) and energy index (EI,) versus
X groups: (a) plotted in group order of ascending BPIxc; (b) a leastsquares line fitted through each set of points. A similar upward trend is seen in both cases, but EIx is not useful as an alternative to BPIxc.
X-H series section). Differences also arise in the BPI ordering of the X-NH2 and X-OH series due to the X = PH2, COCH3, CONH2, CHO, and COOH (also BH2 and NO in X-NH2) groups, with the largest fluctuations occurring in the X-NH2 series. These X are groups for which delocalization of the s electrons is possible thereby causing a partial double bond character of the X-N and X-0 bonds. Thus we hypothesized that if the K electron pathway in these molecules were eliminated the BPI ordering would be the same as in X-C!H3. Choosing the more extreme X-NH2 case, this hypothesis was tested by recalculating the relevant BPI with the geometries twisted so that the lone pair on N was no longer in a favorable position to delocalize: BPIxN = 0.0919, BH,; -0.0172, PH,; 0.0072, COCH,; -0.0066, CONH2; 0.0130, NO; 0.0313, CHO, 0.0291, COOH. Figure 5b gives recalculated BPIxN (indicated by solid circles). The BPIxc BPIxN correlation coefficient becomes 0.973 with X = H included and 0.992 without. The remaining small fluctuations (excluding X = H) arise because the recalculated BPI'S have not employed fully optimized geometries. Also note that the fluctuations in the BPIxo line are smaller than those in BPIxN since the lone pairs on 0 are in a less favorable orientation for delocalization, and partial double bonding plays less of a role in the X-OH series. Thus after compensating for a-bonding effects the CH,, NH2, and OH references produce three parallel curves, the change from carbon to nitrogen to oxygen simply producing a reference level shift.
-
X-H Series The simplest possible reference is the H atom, so BPIxH was calculated for the X-H series and included in Table I. The ordering of groups given by BPIxH is very different from that of
-0.3
(4
-0.2
-0.1
0.0
0.1
0.2
BPI XN (a.u.)
Figure 3. (a) NH2-referencedBPI (BPIxN)versus CH-referenced BPI
(BPIxc). (b) OH-referenced BPI (BPIxo) versus BPIxc. (c) BPIxo versus BPIxN. Here (and in subsequent figures) a least-squares line is fit through the data and a correlation coefficient ( R ) is calculated. The largest deviations are caused by the X = H point, and refitting the data without X = H gives higher correlations ( R = 0.972, 0.986, and 0.980 for a, b, and c, respectively). BPIxc (or BPIxN and BPIxo) as can be seen from the lower correlation between BPIxc and BPIxH (Figure 6a) and the large fluctuations in the BPIxH line when BPI (in ascending order of BPIxc) is plotted versus the groups (Figure 7a). (BPIxH also does not correlate well with the EIx, Figures 6b and 7b). The unsatisfactory behavior of H as a reference follows from its well-known unique chemistry. This is manifest in its ambiguous placement in the periodic table as head of group I or group VI1 or often slightly to the left of carbon and arises because of its small size and lack of core electrons. As discussed by Cotton and Wilkins~n,'~ hydrogen is strongly modified by the atom to which it is attached, an exactly opposite characteristic that is desired as a group electronegativity reference. Unfortunately, the two most promising previous group electronegativity scales, those of Marriott et al.3 and Boyd and EdgecombeS have both employed hydride compounds. It may be noted here that one cited reason for using H as reference in preference to CH, is the possibility of hyperconjugation effects in X-CH,. Study of the electronic (15) Cotton, F. A,; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; John Wiley: New York, 1980; Chapter 6 .
160 The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992 v
-
Reed and Allen
+ 0.44022X R = 0.865
.0.25581
-$
0.2
7
0.1
-
0.0-
m
h
j
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-
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0.5
.
I . .
0.6
.
t
.
.
.
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- El X (a.u.)
I
.
0.8
..
I
.
0.9
.
.
i
1.0
y = .0.31255 t 0.47017~ R = 0.844
-
0.2'I
-0.2t
-0.1
m
1'
*
-0.2
-0.3
0.5
0.6 0.7
0.8
0.9
1.0
v - E l X (a.u.) y
0.2'I 0.1
0.0
I
.0.40017
+ 0.52206~ R
-
0.819
4
-0.2 .0.1
Group
(b)
Figure 5. (a) BPIxc, BPIxN, and BPIxo versus groups in ascending order of BPIxc. The largest deviations occur for X = H and for groups in which *-electron delocalization is possible. (b) BPIxc and BPIxN versus groups. Relevant BPIXN'S were recalculated for altered geometries so that delocalization was no longer possible (indicated by solid circles). y I 2.357%-2 + 1.8528X
. I
A
i
-
R 0.884
..
0.4-
cc
-g
0.2-
x
F m 4. (a) BPIxc versus EI,. (b) BPIxN versus EIx. (c) BPIxo versus EIx. Correlation is poor between EIx and BPI using a CH,, NH2, or OH
reference. structure of our compounds does not indicate that this has an appreciable influence on BPIxc, and our analysis of NH2 and OH as references (discussed above) and comparison to experimental data (given below) substantiates CH3 as the most appropriate reference.
Comparison to Experimental Data Although group electronegativity is not a directly measurable quantity, there are several experimental properties which have been postulated as proportional to it. Thus previous efforts have attempted to correlate group electronegativity scales with 'Jcc coupling constants in monosubstituted benzene~,39~J~ JHH- values in monosubstituted ethenes,2 I3C N M R chemical shifts of the a-carbon in monosubstituted methanes and ethanes,2 and Taft's substituent constants a* and u1.I7 Here we compare BPIxc with five experimental measures: substituent constants due to field/inductive effects determined from substituted benzenes (F and uF),I3C N M R chemical shifts of the methyl carbon in monosubsbtuted methanes (6,) in solution and in the gas phase, NMR coupling constants in monosubstituted benzenes (IJa ipso-ortho), (16) Datta, D.; Singh, S. N. J . Phys. Chem. 1990, 94, 2187. (17) Datta, D. J . Phys. Org. Chem. 1991, 4, 96.
0.0-
-0.2 -0.2
0.1
0.0
-0.1
0.2
BPI XC (a.u.)
(a)
o'6 9
:
y = .0.48668 + 0.87453~ R
I
..
0.4-
Y
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x
3
o.o{ -0.2 - .
'
.
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.
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Application of BPI to Group Electronegativity o.6
1
The Journal of Physical Chemistry, Vol. 96, No. 1 , I992 161 TABLE III: Substituent Constants (Fadup), '%Z NMR Chemical Shifts (&), Coupling Constants (Ipso-Ortho) (lJ-), and Core-Electron Binding Energies ( E B )
6, X
P
uFb solution'
BeH BH2 SiH3 H 0.00 0.00 CH2CH3 -0.02 0.00 -0.01 0.00 CH3 CHiNH2 PH2 CHZOH 0.06 CHCH2 0.22 CH2F 0.50 0.26 COCH, CONH2 0.38 0.14 NH2 CCH 0.41 NO 0.52 0.28 SH 0.3 1 CHO 0.44 COOH OH 0.46 0.30 0.72 0.45 c1 0.64 0.44 CF3 1.00 0.65 NO2 0.90 0.60 CN 0.74 0.44 F (b)
Group
Figure 7. (a) BPIxc and BPIxH versus groups and (b) BPIXH and EIx versus groups for groups in ascending order of BPIxc.
separate substituent constants (R, resonance; F, field/inductive) from measured equilibrium and rate constants for meta- and para-substituted benzenes (later updated and improved by Swain, Unger, Rosenquist, and SwainIEb).Similarly, Taft and Top"'sc have assigned parameters to characterize four kinds of substituent effects in para-substituted benzenes: field, electronegativity, resonance, and polarizability. (Their electronegativity parameter is not an experimentally derived parameter, but an ab initio quantity obtained from a Mulliken population analysis of the H atom and will be considered in the following section.) BPIxc versus the field/inductive parameters F and uF are given in Figure 8 parts a and b, re~pectively.'~Although the inductive effect is believed to be relatively small compared to the field effect, the two parallel each other sufficiently to provide a moderately strong (R = 0.931 and 0.912) correlation, even though BPI is a measure of bond polarity which reflects only inductive effects. Especially encouraging is that BPIxc,. calculated for methyl compounds in the gas phase, correlates well with the substituent constants measured for benzene derivatives in solution, thus suggesting that BPI,c may be generally useful as a measure of group electronegativity. BPIxc versus 6,. The 13C N M R chemical shift of a carbon attached to the X group is often cited as a quantity dependent on the electronegativity of X. Chemical shift values relative to TMS for the methyl carbon in the X-CH3 compounds were obtained from the compilation of Breitmaier and Voelter,20and BPIxc is plotted versus their solution 6, in Figure 8c. Two separate lines depending on whether the attaching atom of X is a row 1 or row 2 atom are apparent. This separation is indicative of the greater polarizability of these groups relative to the average of (18) (a) Swain, C. G.; Lupton, E. C. J. Am. Chem. Soc. 1968, 90,4328. (b) Swain, C. G.; Unger, S . H.; Rasenquist, N. R.; Swain, M. S . J. Am. Chem. R.D.In Progress in Physical Organic Chemistry; Taft, R.W., Ed.; John Wiley & Sons: New York, 1987; Vol. 16, p 1. (d) Bromilow, J.; Brownlee, R. T. C.; Lopez, V. 0.;Taft, R. W. J. Org. Chem. 1979, 44, 4766. (19) The uFvalues used here come from those tabulated in ref 18c; however, these values do not differ significantly from the set of ul that Taft et al. published previously,'" and the latter did not attempt a separation between field and electronegativity components. (20) Breitmaier, E.; Voelter, W. C-13 NMR Spectroscopy, 3rd ed.;VCH: Weinheim, 1987.
Soc. 1983,105,492. (c) Taft, R. W.; Top",
-2.3 16.1 6.5 19.0 -4.4 17.6 19.4 13.3 30.1 28.3 6.5 30.9 20.3 49.9 25.6 61.4 0.3 71.6
gas phased
0.0 -24.3 -14.3 -19.9 -26.2
lJoCc
-6.50 0.0 1.09 1.07 1.42
5.20 3.37
-33.3
4.20 2.00
-78.9
290.4 290.9 290.7 290.78
1.65 291.1 1.61 2.00 291.2 1.90 291.2
1.89 -30.4 -7.3
-5 1.4
E$
291.8
291.4 291.4 29 1.6 9.70 292.3 9.21 292.4 3.57 292.1 11.43 293.0 4.11 292.7 14.84 293.6
"Reference 18a,b. For meta- and para-substituted benzenes in solution. b Reference 18c. For para-substituted benzenes in solution. 'Reference 20. X-CH3 chemical shift measured in ppm relative to TMS. dReference 21. X-CH3 chemical shift measured in ppm relative to CH4. CReference3. For monosubstituted benzenes; determined as 80% solutions in (CD,),CO; measured in Hertz relative to H. fReference 22. Gas phase C 1s binding energy of carbon in X-CH,; measured in eV. SA value of 285.0 eV is given in the literature for the solid. A scale factor of 5.7 (determined by comparison of EB for species where both gas phase and solid data were available) was used to obtain an estimate of EB in the gas phase.
the row 1 groups-a significant component of the chemical shifts, but not part of group electronegativity. The points X = CH2F, COOH, and C N are also displaced upward from the row 1 line. The gas-phase 6, data2I are plotted versus BPIxc in Figure 8d. Values for row 2 or CH2F, COOH, and CN groups are not available in this case, but the large deviation for the X = C C H point (which is not present in the solution 6, plot) and the small data set of gas-phase experimental values remind us that the good correlation in this case is probably fortuitous and does not imply a basic physics relationship. BPIxc versus IJCe. 'J, (ipso-ortho) coupling constants for monosubstituted benzenes were obtained from ref 3 and plotted versus BPIxc in Figure 8e. The least-squares line and low correlation coefficient (0.847) demonstrate that no meaningful correlation exists. BPIxc versus EB. The most important experimental measurement to be compared to BPIxc is the C l s binding energy of the methyl carbon in X
