NMR chemical shifts and intramolecular van der Waals interactions: carbonyl and ether systems

Journal of Molecular Structure, 190 ( 1988) 99-l 11 Elsevier Science Publishers B.V., Amsterdam - Printed

99 in The Netherlands

NMR CHEMICAL SHIFTS AND INTRAMOLECULAR VAN DER WAALS INTERACTIONS: CARBONYL AND ETHER SYSTEMS*

D. B. CHESNUT**,

D. W. WRIGHT

and B. A. KRIZEK

P. M. Gross Chemical Laboratory, Duke University, Durham, NC 27706 (U.S.A.) (Received 25 January

1988)

ABSTRACT The correlation observed between chemical shifts and local intramolecular van der Waals energies as determined by molecular mechanics is extended to carbon systems containing carbonyl groups or ether linkages. The correlation is maintained for not only non-carbonyl carbons in such systems but also for carbonyl carbon and oxygen atoms and ether oxygen atoms as well. Comparison of our shift correlation with work involving a similar correlation with optical transitions provides a firmer base for the model and also suggests a possible reinterpretation of the underlying theory. Unusual effects are observed for carbons /I to the carbonyl group which cannot as yet be explained in terms of either flaws of the force field model or available experimental and theoretical information on such systems.

INTRODUCTION

Force field or molecular mechanics approaches have shown great promise in recent years as a method of studying molecular structure using an approach in which the various atomic interactions are modeled by relatively simple analytical potential functions. The MM2 and MMP2 force fields due to Allinger [ 11 are perhaps some of the better known force fields that are generally applied to the study of organic molecules ranging in size from a few atoms to several hundred. Nuclear magnetic resonance (NMR) has for a long time been a key experimental probe of molecular structure. The chemical shift of a nucleus is a very sensitive measure of the dependence of the resonance field of a nucleus on its particular molecular environment. The authors and co-workers [2-41 have recently uncovered a linear relationship between the local van der Waals energy for nucleus i, EvdW,i, and its chemical shift, ~31~‘;the relationship associates a repulsive van der Waals interaction with deshielding effects, and an attractive van der Waals interaction with shielding effects. One is led to a conceptually simple physical rationalization of the beta, gamma, and delta ef*Dedicated to the memory of Professor Walter Gordy. **To whom correspondence should be addressed.

0022-2860/88/$03.50

0 1988 Elsevier Science Publishers

B.V.

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fects in NMR. The results seem to be generally applicable to a variety of nuclei; to date the method has been applied to saturated aliphatic phosphines (phosphorous), amines (nitrogen), hydrocarbons and monofunctional alcohols (carbon [2] andoxygen [4]), and monofluorinated hydrocarbons [ 41 (fluorine). Root-mean-square errors between calculated and observed shifts are 11, 6, and 3 ppm for phosphorus, nitrogen and carbon systems, respectively, and are 10 and 17 ppm for oxygen and fluorine, respectively; although the r.m.s. error varies for the different nuclei, these nuclei have different ranges of shifts and the errors correspond roughly to 8-12% of the shift range in each case. Li and Allinger [5] have recently extended the correlation to hydrogen in some rigid cyclic ring systems. The intramolecular van der Waals interaction as defined in conventional force field calculations is an attempt to mimic the long-range attractive and repulsive non-bonded interactions in a system. The fact that this term empirically is a reflector of the chemical shift of a nucleus is an important correlation relating experiment and structure. In the present study we extend this correlation to that very important class of oxygen-containing compounds consisting of aldehydes, ketones, acids, and ethers. Chemical shift data for both carbon and oxygen are analyzed and are shown to display the same linear relationship as previously observed for other systems. As might be expected, most of the data for carbons distant from the carbonyl group are very much the same as previously observed, while both the carbonyl carbon and oxygen atoms are studied here for the first time. In addition, some unusual and as yet not understood proximity effects are reported. The addition of the ether-containing or carbonyl-containing systems to the class of molecules which exhibit the linear correlation is an important extension of our past studies, and continues to suggest that this relationship is most likely universal in its application to all chemical species. CALCULATIONS

In the present study we continue to use the force field calculations represented by the MM2 program of Allinger [l] as received from the Quantum Chemistry Program Exchange [6] and contained in the program MODEL of Still [7]. The molecular mechanics steric energy contains terms for bond stretching and compression, bond angle bending and bend-stretch effects, l-4 torsional interactions, 1-4 and greater van der Waals interactions, and dipole-dipole or charge-charge interactions. The van der Waals interaction in the program is based on the Hill [8] form of the Buckingham and Ulting potential [9], a potential virtually identical with the Lennard-Jones 6-12 type. By minimizing the total steric energy of a molecule, the molecular mechanics program determines the optimum geometry, the individual contributions to the steric energy from each subset of atoms, and the total steric energy. We

101

look specifically at the local steric energy of the resonant nucleus under consideration and, in particular, at the van der Waals contribution, &w, to this energy. Isotropic chemical shift data are reported according to the parameter 6

where the reference compounds are TMS for carbon and Hz0 for oxygen. The usual theoretical definition of the shielding constant ais used in which positive values indicate diamagnetic or upfield shifts, and negative values paramagnetic or downfield shifts; correspondingly, 6 values are such that positive values correspond to paramagnetic or downfield shifts relative to the reference, and negative values to diamagnetic or upfield shifts relative to the reference compound. The present study includes carbon data taken from Sadtler [lo] for 23 carboxylic acids, 13 aldehydes and 35 acyclic ketones, from Weigert and Roberts [ 111 for 16 cyclohexanones, and from Breitmaier and Voelter [ 121, Sadtler [lo] and Stothers [ 131 for 18 ethers. Oxygen data for 9 aldehydes, 33 ketones (acyclic and cyclic) and 4 carboxylic acids were taken from Kintzinger [ 141 and for 10 ethers from Beraldin [ 151 and Kintzinger [ 161. Although the other steric terms may make contributions to the chemical shift correlation, a statistical study employing the backward elimination method (as outlined in Draper and Smith [ 171) shows that the van der Waals energy is the dominant term; in addition, single term correlation coefficients show the van der Waals term to be the most correlated. Inclusion of other energy terms in the regression analysis results in only a small reduction in the r.m.s. error. Accordingly, we continue to use the van der Waals term alone in our correlation studies. Multiple regression analyses including standard deviations and tstatistics were carried out assuming a shift equation for resonant nucleus i of the form d@’ =bk +c VdWE vdW,i L

(1)

where k specifies the class of the resonant nucleus, bk is an associated class or bonding constant, and &dw,i is the local van der Waals energy associated with nucleus i. For carbon a variety of class constants is needed according to whether the atom is a primary, secondary, tertiary or quaternary carbon, and what position it occupies with respect to the carbonyl group (vide infra). The class constants, i.e. the intercepts, were checked for equivalence using the method outlined in Brownlee [ 181. Unique parameters are also needed for the three types of carbonyl carbon atoms. The class constants for the 7 largest classes and c,dw were determined in a single multiple regression; thus, although some classes may exhibit different slopes if fitted individually, our approach has been to use a common slope in the interest of simplicity. The various carbonyl class constants were deter-

102

mined similarly. The remaining class constants were determined (as calculated above) and individually optimizing each intercept. calculated shifts reported in this paper use eqn. ( 1).

using c,dw All of the

RESULTS

As indicated in eqn. (1) , the experimental chemical shift is correlated with the local van der Waals steric energy of the appropriate nucleus in a linear way. Different classes of nuclei have different class constants ( bk), while in our treatment all classes have been constrained to have a common slope ( cvdw ) . This relationship is illustrated in Fig. 1 for three different classes of secondary carbon nuclei in ketones; the plot of calculated versus observed shifts for the same set of nuclei is shown in Fig. 2. The r.m.s. error for carbon is typically of the order of 2-3 ppm. Although improved agreement would be obtained by removing the common slope constraint, we keep that constraint in order to maintain the simplicity of the model at its lowest possible level. Our results, then, consist of a tabulation of intercepts for various classes of nuclei, and slopes for the various species involved. These are given in detail in Tables 1 and 2 which we shall discuss briefly. A great deal of data is contained in Tables 1 and 2. What we shall try to do is to point out the general trends observed, the agreement of current results with those obtained previously for other types of systems, and several of the unusual effects observed in the present study. By far the largest group of molecules studied (342 resonant nuclei) are those singly bonded carbons in alde-

a

30

Observed

50

Shifts

Fig. 1. Carbon-13 chemical shifts for the secondary carbon atoms of ketones as a function of their local van der Waals energy: 4, secondary carbon atoms a to the carbonyl group; A, secondary carbons in the position /3to the carbonyl group; 0, secondary carbons occupying neither type of special position. Fig. 2. Calculated and observed shifts for the ketone secondary carbon atoms displayed in Fig. 1. The solid line represents the 45’ line between calculated and observed data.

103 TABLE 1 Class constants (ppm) for the carbons of acids, ketones (ket), and aldehydes (ald) as a function of the immediate carbon environment (primary, secondary, tertiary, or quaternary) and its location ((Y or /3) relative to the carbonyl group”

acid ket ald

acid ket ald

pri-/3

pri

pri-cu

2.5(l) -1.2(12)

10.4(112)*b

11.9(l) 30.6( 13) 31.2(l)

set-p

set

set-ol

22.0(102)*’

22.4(13)* 36.0(52)* 40.6(7)

ter

ter-or

28.9(11)

32.3(7) 43.5(17)* 47.4(5)

qua

qua- cx

33.8(4)

30.6(2) 42.4(6)

c

16.6(14)* 12.3(32)* 1

I ter-/3

acid ket ald

24.1(3) 19.1(7) 1

c qua-D

ket

31.9(l) 34.6(l)

Statistical equivalent classes are indicated by brackets; the number of data points are indicated in parentheses. Standard deviations of the class constants are typically 0.5-1.0 ppm. Classes indicated by an asterisk were included in the major multiple regression determining the van der Waals’ slope (see Table 2 ) . “16 Primary non-adjacent ether carbons included. ‘12 Secondary non-adjacent ether carbons included.

hydes, ketones, carboxylic acids, and ethers. Most of the intercept data for these species are given in Table 1 and are tabulated according to type of molecule, type of carbon atom (primary, secondary, tertiary, and quaternary) and whether or not the particular carbon is cr or p to the carbonyl carbon or further away. Those cases where different types of molecules yielded statistically different intercepts are indicated by brackets; thus, for example, all primary carbons for acids, ketones, aldehydes, and ethers are equivalent, whereas for primary-p species the acids are different from the other groups. Those (large) groups of nuclei which were involved in the large multiple regression (that determined the reported slope) are indicated by asterisks. The number of data points for each class is indicated in parentheses. As explained previously, the intercepts for those classes with small numbers of data points were determined by forcing the multiple regression slope on this set and optimizing the inter-

104 TABLE 2 Class constants,

bk, and van der Waals’ slopes, cVdw,for systems currently

System

N

1 Carbon (a) Non-carbonyl aldehydes, ketones, acids and ethers

5k (ppm)

studied

cvdW

(ppm (kcal mol-‘)-I)

R.m.s. error (ppm)

342b

(see Table 1)

29.4(1.1)

3.0

(b) Secondary-adjacent

7 11 6 3

58.0 (0.6) 49.0(1.4) 64.8(0.6) 59.4(M)

29.4( 1.8)

1.2

(c) Acid carbonyl carbon

23” 13 47

178.8(0.3) 200.4(0.4) 207.2 (0.3)

5.9(0.4)

1.3

100.4(6.1)

9.1

2.4(3.7)

9.9

alcohol Secondary-adjacent ether Tertiary-adjacent alcohol Tertiary-adjacent ether

Aldehyde carbonyl carbon Ketone carbonyl carbon 2 Oxygen (a) Ether oxygen Alcohol oxygen (b ) Acid carbonyl oxygen Aldehyde carbonyl oxygen Ketone carbonyl oxygen

10 20 4 9 34

-47.4(5.1) -22.7(3.2) 251.5(5.2) 585.2(3.6) 556.6(3.5)

“The number of data points for the various groups (N) is indicated as well as the r.m.s. error. The data standard deviations are shown in parentheses. bSeven classes. “Three classes.

cepts. Some of the data are represented by only a few points but are reported here for completeness. The class constants (intercepts), bk, are an attempt in our simplistic model to relate the major local shielding effects, and reflect very much the different bonding situation in which a particular nucleus finds itself. Thus, in previous work [ 21, the fact that singly bonded primary, secondary, tertiary, and quarternary carbons showed different class constants was taken to simply reflect the fact that a carbon-hydrogen bond is different from a carbon-carbon bond. This idea has its limitations, however, in that in cases where significant strain is involved new classes must be defined [3]. It is not surprising, then, that those carbons a! to the carbonyl group are different from those further removed, since the a-carbons find themselves bound to a carbonyl moiety with its associated n system as opposed to being bound to an ordinary singly bonded carbon. The different classes, then, for a-carbons are not an unexpected result. For aldehydes and ketones the class constants increase by 14-20 ppm in going from an “ordinary” environment to a position cy to the carbonyl carbon. On the other hand, an unexpected result in our study is that those carbons p to the carbonyl carbon must also be placed in separate classes. This effect

105

can best be seen by example in Fig. 1 where clearly the secondary carbons in the p position differ from those further removed. This /3-carbonyl proximity effect results in a decrease of the class constant from its “normal” value by some lo-12 ppm, and occurs whether the carbon in the /3 position is primary, secondary, or tertiary; there is insufficient data to make a judgement about the quaternary case. Table 2 contains information on slopes, r.m.s. errors, class constants, and the number of nuclei studied for other systems in the current study. As was the case for alcohols studied previously [ 21, there is a difference between carbons which are adjacent to the OH moiety (adjacent carbons) and those that are remote. This is again born out in the study here involving ethers in that both secondary and tertiary carbons which are adjacent to the ether oxygen are different from their secondary and tertiary remote counterparts. In addition, it is found that there is a statistical difference between ethers and alcohols in this regard, with carbons adjacent to the ether oxygen having a class constant lower by some 6-10 ppm than those adjacent to the alcohol oxygen. Part 2 (a) of Table 2 shows, however, that the oxygen slope is uniform for both ether and alcohol oxygens although, again, as might well be expected, the two types of molecules have different class constants (different local environments). Of particular significance in this work is the study of the carbonyl carbon and oxygen atoms with regard to the correlation between the chemical shifts and the local van der Waals energy. The data for carbonyl carbons is shown in Fig. 3 where one can clearly discern the three different classes of carbonyl containing compounds (acids, ketones, and aldehydes); calculated versus observed shifts are shown in Fig. 4. Likewise, three classes occur for the oxygen atom of the carbonyl moiety. The significant difference in the carbon and

Fig. 3. Chemical shifts as a function of the local van der Waals energy for carbonyl carbons in ketones (+ ) , aldehydes ( A 1, and carboxylic acids ( 0 ) . Fig. 4. Calculated and observed chemical shifts for the carbonyl carbons shown in Fig. 3. The solid line is the 45 o line between calculated and observed shifts.

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oxygen results is that the oxygen dependence upon the van der Waals energy is statistically flat whereas that of the carbon atom has a statistically non-zero slope. It is of interest to note that the sensitivity to the van der Waals energy of the carbonyl carbon is very low, much lower than that for singly bound carbon or oxygen, and, indeed, is the lowest non-zero slope value that we have found to date. The carbonyl carbon data is of special interest with regard to a recent correlation found by Cornish and Baer [ 191 (vide infra) which relates chemical shifts of the carbonyl carbon in cyclic ketones with the n+3s electronic transition. DISCUSSION

The present work has extended the correlation between the local van der Waals energy calculated from molecular mechanics and the chemical shift of a pertinent nucleus to carbonyl and ether systems. The singly-bound carbon atoms yield parameters which, as might be expected, are in good agreement with those determined previously for simple aliphatic hydrocarbons and monofunctional alcohols. The present work has extended the correlation to the carbon and oxygen atoms of the important carbonyl group as well as the ether oxygen atom. The model is a useful one in that in a fairly simplistic way c~, p, y,... effects are described with a single approach which relates the shift not only to the local structure but to the general conformation and long range structure of the molecule. In addition, there are two further interesting aspects of our study, one relating to the unusual effect observed for carbons j? to the carbonyl group, and the other relating to a possible reinterpretation of the theoretical base of the model. We move to discuss these two items at this point. As has been pointed out before [2,3], when atoms are located in highly strained positions deviations from our simple model occur. However, the effect observed here of the unusual change in the class constant for carbon atoms j? to the carbonyl group cannot be explained on this basis. Primary, secondary, or tertiary carbon atoms in this location show a reduction in their class constants (increased shielding) of about 8-9 ppm below that of their “normal” counterparts. Carbons in this position are somewhat unusual in that they tend to eclipse the carbonyl oxygen in the lowest energy conformation of the molecule. Stability of CH eclipsing the carbonyl group was shown in the very early work of Kilb et al. [ 201; extensive references to the stability of carbon-carbon bonds in the eclipsed position are given in the papers by Allinger et al. [21] and Suter [22], and were used in the parameterization of their force fields. Wiberg and Martin [ 231 have presented arguments suggesting that the stability of this form is aided by a dipole-induced dipole interaction between the carbonyl group and an induced dipole in the C-C bond linking the cy and j? carbons; they exhibit calculations which show a shift of charge from the carbon p to the carbonyl to that carbon cy to the carbonyl under the influence of the carbonyl dipole moment. An argument based on the effect of charge depletion

107

or addition to a particular carbon would be extremely useful here but is clouded by the controversy over the polarity of the CH bond. Arguments involving Mulliken population analyses indicate carbon to be more electronegative than its bound hydrogen [ 241, and that as there are fewer hydrogens attached to the carbon the net negative charge associated with the carbon would decrease; since the class constants observed in this and prior work tend to increase with the number of carbon-carbon bonds associated with the resonant nucleus, the argument based on Mulliken charges leads to an incorrect prediction. On the other hand, Wiberg and Wendoloski [ 251 have argued that the polarity of the C-H bond should be that with the negative charge on hydrogen; such an argument would suggest that as the number of carbon-carbon bonds is increased the net positive charge on carbon should decrease; this argument would be in agreement with the effect observed here given that the carbon p to the carbonyl is depleted in charge and its associated “observed” class constant reduced. A difficulty still remains, however, as is shown by the calculations of Wiberg et al. [ 261 which are very sensitive to the basis set employed and, indeed, show a reversal of the direction of charge decrease when one moves from a 6-31G** to a 6-31G* basis. These authors indicate that the fully polarized 6-31G** basis is probably more accurate in that it is a balanced basis, but the effect seems extremely sensitive and at this point somewhat uncertain as to its proper determination. It is attractive to note that, given the C-C polarization induced by the carbony1 group, the class constants of the a- and P-carbon atoms move in opposite directions, as do the charges on the two types of atom. However, part, if not all, the effect on the class constant of the a-carbon may well be due simply to its proximity to the carbonyl group itself. The situation is further clouded by effects seen in a study of a small subset of cyclohexanones. p Carbons formed from substitution at the (x ring position do indeed share the anomalous /I?class constant shift effect; these substituents are forced to be eclipsed with the carbonyl oxygen due to the ring geometry. /3 Carbons in the cyclohexane ring, however, do not show the effect and behave basically as “normal” secondary (for example) carbon atoms. These carbons in the ring, again by the nature of the ring structure, are not eclipsed with the carbonyl oxygen. Finally, carbons in the ring at the y position to the carbonyl carbon do show this anomalous effect, again with approximately an 8-9 ppm reduction in the associated class constant, the same as that observed for /?carbons in acyclic systems. Clearly, a more fundamental understanding of the relation of the present model to quantum mechanical reality must be had before these unusual and unexpected proximity effects can be understood. In order to determine if these proximity effects might be real or possibly an artifact of the model an approach was taken whereby the oxygen of the carbony1 group and its lone pairs are treated as separate entities (in MM2 lone pairs are not usually placed on carbonyl oxygens). Given the minimized energy structure from the normal molecular mechanics calculation, van der Waals

108

interactions at these positions were calculated analytically using for the carbony1 oxygen the alocholic oxygen van der Waals parameters, and placing two lone pairs at appropriate distances from the oxygen at an angle to each other of 120” in the carbonyl group plane. Our data indicate that carbons at the p position are shifted approximately 10 ppm below that of a “normal” secondary carbon, corresponding to approximately a 0.3 kcal mol-i higher repulsion. Employing the approximate treatment described above with the lone pairs shifts the so-called P-line in the proper direction that would make it “normal” but only by approximately 0.03 kcal mol-‘. In a similar manner, secondary carbons at the cyposition are observed to be shifted approximately 20 ppm higher, about 0.7 kcal mol-’ (more attractive) than the “normal” secondary line. The treatment above does indeed shift the a-line down toward the “normal” line but again not by a significant amount, only some 0.1 kcal mol-‘. Although our treatment is approximate, it does not seem likely that the explicit inclusion of lone pair electrons on the carbonyl oxygen can resolve the problem. The question of the role of the carbonyl n electrons is a moot one since they do not appear explicitly in the MM2 force field treatment. The correlation between the local van der Waals energy and the chemical shift of the pertinent resonant nucleus associates a deshielding effect with repulsive van der Waals interactions and a shielding effect with attractive van der Waals interactions. This can be simply illustrated by considering the butane molecule and a terminal methyl carbon as the resonant nucleus. Interactions between the terminal carbon and the beta (to the terminal carbon) carbon hydrogens are in that strongly repulsive range of the van der Waals energy, and provide an explanation of the well known beta effect in NMR; the terminal carbon at the other end of the molecule and its associated hydrogens are further away, in the attractive range of the van der Waals potential, and give rise to a net small attractive interaction and, thus, a small shielding effect, the well known gamma effect in NMR. The ad hoc rationalization of our prior work has focussed on the dominant negative paramagnetic term in the expression for chemical shielding and, in particular, on its dependence on r-3. The model proposed was one in which repulsive interactions caused a compression of the orbitals about the resonant nucleus causing r to decrease, r -’ to increase, leading to a deshielding or paramagnetic effect on the chemical shift. Conversely, attractive interactions were pictured as giving rise to an expansion of the orbitals which, by the same argument, would then give rise to an algebraic increase in the paramagnetic term, a net shielding effect. This ad hoc rationalization is, at the moment, no more than a picture whereby the effect can be remembered, there is as yet no firm basis in theory for this picture, or, for that matter, any other. The other types of terms which occur in the paramagnetic portion of chemical shielding are energy terms in denominators (the difference of energies of the ground and excited states which, via perturbation theory, are coupled). Qualitatively speaking, effects which would give rise to a decrease of

109

an effective excitation energy would enhance the (negative ) paramagnetic term and lead to deshileding effects, while interactions which would cause this energy to increase would give rise to shielding effects. Cornish and Baer [ 191 have recently reported some extremely interesting results concerning the shift in the n+3s optical transition energy of methyl cyclopentanones and cyclohexanones, as well as some branched chain and bicyclic ketones. They find that substitution of alpha methyl groups in the cyclic systems give rise to a red shift in this particular transition, while substituents at the beta position yield a blue shift. Arguments are presented which indicate that the excited states in question are relatively insensitive to these substitutions, so that the red shift observed upon methyl substitution at the alpha position is actually due to an increased repulsion in the ground state; likewise, the blue shift is a result of attractive interactions involving the substituted methyl group at the beta position. Cornish and Baer [ 191 found that they could correlate their optical shifts with 13C NMR chemical shifts, and indeed, noting some of our own work, justify their proposed effect on the ground state in terms of van der Waals or nonbonded repulsions or attractions. Furthermore, the common correlating factor in the chemical shift allows them to plot their optical transition energies against our calculated (via molecular mechanics) van der Waals energy to obtain a linear relationship, as expected. The Cornish and Baer slope of energy versus chemical shift is approximately 1.5 ppm (kcal mol-‘) -’ compared to the result reported in this paper of 5.9 ppm (kcal mol-l) -l. The scatter of the data is sufficiently small so as not to allow this as a possible explanation of the difference in the two results. It should be recalled that there are apparently two different processes in action here. The chemical shift of the carbonyl carbon, for example, is somehow being determined by the local van der Waals interactions as seen by that nucleus; the optical energy shifts measured by Cornish and Baer [ 191 are probably reflecting these nonbonded interactions as seen by the CO moeity as a whole. Cornish and Baer point out that the CO excitation is also probably sensitive to stress (angular) perturbations as well as the van der Waals interactions, although these two effects would likely parallel each other. Although this particular optical transition is a very special and localized one, and although Cornish and Baer [ 191 have correlated only the 13C chemical shift of the carbonyl carbon and the optical energies, our own work encompasses not only the carbonyl moeity (as reported in this paper ) but also general carbon (and other nuclei) systems where no such special n+ 3s transitions are defined. The suggestion is, then, that perhaps it is modifications to the energy terms in the chemical shift expression which are responsible for the correlation of the NMR shift and the van der Waals attractions and repulsions. For this to be true in general, one would have to accept the premise that ground state energies in general are more sensitive to internal perturbations than those of excited states. That is, the addition of groups which give rise to attractive or repulsive van der Waals interactions with other parts of the molecule would

110

cause decreases or increases of the ground state energies of the pertinent fragment with little, if any, changes in excited state energies. This possible aspect of the present correlation is certainly intriguing and deserving of further attention. It serves to remind us again how empirical correlations can often point the way to significant theoretical investigations. While one can correlate the carbon chemical shift with the optical transition energy, there is no reason not to look at whether or not a similar correlation exists for the same transition and the carbonyl oxygen shifts. Our calculations of the relationship between the oxygen chemical shift and the van der Waals interactions indicates that it is statistically flat; that is, the oxygen chemical shift is basically insensitive to the van der Waals effects. Taking Cornish and Baer’s data [ 191and analyzing it against the carbonyl oxygen chemical shift produces a similar result; namely, the slope between their optical energies and the oxygen chemical shift is statistically zero. It seems unusual that the basic source of the nonbonded electrons involved in the optical transition being “on” oxygen would not be as sensitive a probe to a chemical shift as the adjacent carbon atom; on the other hand, the slope relationship between the two effects may be zero for causes yet uncovered. ACKNOWLEDGMENT

D.B.C. is very pleased to contribute a publication to this in memoriam issue honoring Walter Gordy, and to acknowledge his influence on his scientific career. It was a privilege to know Walter not only as a post-doctoral research associate but later as a colleague and friend at the same university.

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13

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