Empirical NMR Chemical Shift Correlations for Methyl and Methylene Protons Edwin C. Friedrich and Katherine Gates Runkle University of California, Davis, CA 95616 Chemical shift correlations are of considerable value because the NMR chemical shifts of selected nuclei for a Iarse variety of compounds can be predicted conveniently hy simple calculations. One of the earliest of these was that presented by Shoolery ( 1 , 2 )in 1959 for calculating the chemical shifts for methylene protons. This correlation, given below in slightly modified form (eqn. I), where ox and oyareconstants for the substituents X and Y, has received such widespread attention that it is included in most trxts fur reaching intervretatiun of 'H N M K spectra. Shoolery originally provided a list of 10 substituent constants to be used with the relationship, and a limited number of additional values have been added by others over the years. Thus, the fourth edition of Silverstein, Bassler, and Morrill's text (3), which was uublished in 1981, lists 23 substituent constant values. little information i.; available regarding the sources of most of the whntituent constants or of their validity and internal consistency ( 4 , 5 ) . Table 1 presents an internally consistent set of 63 suhstituent constants which we have developed (6)for use with the Sboolery Relationship (equ. 1) to predict the chemical shifts for methylene protons of acyclic compounds in dilute CC,bor CDC13 solutions. The chemical shift data used in deriving the constants were taken mainly from primary sources of quality 'H NMR spectra (7). When using the substituent constants, excellent agreement between observed and predicted proton
ow ever,
chemical shifts is found for a large variety of different methylene derivatives. This is illustrated for a number of the most commonly encountered substituents by the comparisons given in Table 2. Note that by treating hydrogen as a suhstituent with a constant of 0.34, the substituent constants may also be used satisfactorily for calculating the proton chemical shifts of methyl derivatives. Of the approximately 100examples ofdifferent methyl or methylene derivatives whose otiserved and calculated chemical shifts were compared during our work, 62% were within f0.1 ppm, 82%within f 0.2 ppm, 92% within f 0.3 ppm, 96% within f 0.4 ppm, and 99%within f0.5 ppm. This distribution of discrepancies is closely similar t o that observed by Matter and co-workers (8) (94% within f0.3 ppm) with use of their correlation for calculating chemical shifts of olefinic protons, and illustrates the value of the correlation. For many of the comparisons having large deviations which were encountered in the Present work. the observed chemical shifw were taken from n k e r i c a l tahu'ln~~ons in the literature rather than directly from svectra ( 7 ) .I t is felt that these deviations are most likely duk to misprint, chemical shift calibration, or interuretation errors rather than to failures of the relationship. 1n-fact, in a number of cases where large discrepancies were encountered, further checking showed that the literature values initially used were in error. In determining the constants listed in Table 1, various limitations were placed on the natures of certain of the suhstituents. Specifically, for the "R" group only short, straight-chain alkyl groups and for the ZCHz group only Z = ~
Table 1. Shoolery-Type Chemical Shlfi Substltuent Constants a SubStiuent HsC R ZCH..Z = CI.Br
&
830
r
Subsliiuent
d
0.68 0.58 0.91
HO RO CaHsO
2.5Bb 2.3Bb 2.94
0.66
R(HU(=O)O
3.01
Journal of Chemical Education
Substituent
o
R(H)S ArS RSS CH&(=O)S
1.64b 1.90 1.72 1.94=
~~~
Table 2. Observed * and Calculated" Chemtcal Shlfls of XCH.Y Protons tor Sclected Methylene Dertvatlves a
0.34
0.68
Subst.
H
H3C
0.58 R
0.91
1.83
1.32
ZCH2
CeHs
(=C
1.44
C=C
1.59
N=C
1.46
2.19
ROC(=O)
I
Obierved chemical shins are given In mlts of ppm mwnfleld from imernal standard MerSl for samples run in CCl, a CDCIS e o l u t l ~and were mainly taken lmm p r l m w swrces (0 01 quallv 'H NMR s p e w . 'me values in parmheses were calculated using he quation: 8 ppm = 0.23 0. o,.
+ +
C1 or Br were initially allowed. For the case of the R group, further examination of thirteen isohutyl and seven neopentyl derivatives clearlv demonstrated (.6.) that the suhstituent constant of 0.58is applicable for bulky as well as small alkyl enuos. For the "ZCH."eroun. the resultseiven inTable 3 ilk s t r i t e that the 0.91 .&tiiuint constantis satisfactory for use with a laree varietv of electroneeative -erouos . Z. The sensitivities of thv phmyl and vinyl group substituent constants of 1.83and 1.32. resoectivelv. to substitution on the ring or double bond were &o &aminedhriefly. From a limited that meta or para suhnumber of e x a m ~ l e s( 6 ) .it aimears .. stituents on the benlene rmg or suhstituents at carbon 2 on the vinvl mouos have onlv minor effecw on the chemical shifts of attacGd methylene protons. However, electronegative suhstituents attached ortho on the benzene ring or a t carbon 1on the vinyl group do produce downfield shifts of adjacent methvlene Drotons bv 0.2 to 0.4 m m . ina all^, ii is impo&nt to n o t e s a t if one condenses the list of suhstituent constants given in Tahle 1 t o a minimum number of easily portable, easily memorized values, the short list given in Tahle 4 results. There are all rounded off to the nearest 0.1 ppm and are only given in a form for use without the 0.23 Ppm additivity constant. Thus, they are t o he added directly give the desired chemical shifts (9). In deciding upon the magnitudes of the suhstituent constants for use in the list in Tahle 4, a combination of averaging of the values given in Tahle 1 plus consideration of the estimated frequency with which the suhstituents might he encountered was employed. Although calculations using these values in most cases will give chemical shifts which are reliable only to f0.5 ppm, they still should he very useful for rough estimates of chemical shifts and as a handy guideline for students just learning how to interpret 'H NMR spectra, since interpretation of 'H NMR spectra never relies solely on chemical shifts hut also includes consideration of relative peak areas, spin-spin splitting multiplicities, and coupling constant magnitudes. ~~~
Table 3. Deiermtnation of the Sensltlvlty of the ZCH2 Substltuent Constant to Changes In Z
~
Compound ZCH2CH2CH2Z
CI Br
I C& CH3C(=O)0 HS N=C CHsOC(=O) CIC(==O) HIN DO in D.0 HOC(==O) in OMSOd.
Observed 6
Urwr =
( P P ~for ) Protons on CI
- 0 23112
2.21 2.36 2.33 1.97 1.99 1.89 2.06 2.05 2.13 1.65 1.78 1.80
P~PP~)
0.99 1.06 1.05 0.87 0.88 0.83 0.93 0.91 0.95 0.71 0.78 0.79 Average = 0.90 *0.11
Table 4. Brief Llst of Chemical Shffl Substituent Constants for Use in Calculating Chemical Shins of XCH,Y Type Methylene Protons Atom 01 Attachment for
Substituant
Substituent Constant. oom'
H
c 1sP9
C (sp2or SP) 0 (all) N (except 0 2 N = 3.5) S (except 02S = 2.6) F CI. Br. I Theseare to beadded dlrenly toglvethe calculated chemical shin in ppm downfield TMS.me ~alculatedvalues in mst cases should be accurate to M . 5 ppm a bener.
tmm
Volume 61 Number 9
September 1984
831
Literature Cited I1i Shmierv .I. N.. "Ysrian Aanaiatps Technical information BuUetin." Vol. 2. No 3. Palo
832
Journal of Chemical Education
eraellent ~ d twhen a used in celculati chemicalahirtsofpmtomtoon methin.m t ~ b o b ~ specificallywhere s t least two of the sulmtituentsare electron withdrawing. (6) Runkle. K. G.. M. S. Dissertation.IlniwrriivnfCalifnmia "ads 19111
