INDO-MO study of γ-CH formyl-proton couplings in radicals from vinyl formate

JOURNAL

OF MAGNETIC

RESONANCE

INDO-MO

48,

265-271

(1982)

Study of y-CH Formyl-Proton Couplings in Radicals from Vinyl Formate

KERRY K. KARUKSTIS Paul

M.

Gross Chemical Duke University.

AND

PETER

Laboratory. Department Durham, North Carolina

Received

December

SMITH* of Chemistry, 27706

4, 1981

As an extension of our earlier comprehensive INDO-MO-SCF investigation of a series of simple singly a-substituted aliphatic formate radicals of general formula cH(X)-0-CHO, where an (X-) group is an H- or a substituent such as CH,-, we carried out a similar study of the vinyl formate addition radicals =CH(CHrNHr)-OCHO and =CH(CHrOH)-0-CHO for which y-CH formyl-proton coupling-constant values taken at approximately 25°C in aqueous solution are now available. The present study shows that the INDO-calculated y-CH formyl-proton coupling-constant values for these two radicals fit very well the general pattern found in our previous INDOMO-SCF treatment of simple a-formyloxy radicals and, together with the experimentally determined values for these same radicals, allow us to assign their preferred conformations. These preferred conformations are in general agreement with those previously assigned to other KJH(X)-0-CHO radicals in our earlier work. INTRODUCTION

Earlier EPR studies (1-4) of simple cu-formyloxy radicals of structure 4Z(X,)(X,)OCHO, 1, where an (X-) group is an H- or a substituent such as CH3-, demonstrate that, in general, these species exhibit r-CH formyl-proton couplings which depend on the nature of the substituents at C,. Using comprehensive INDOMO-SCF calculations (5) on a number of type-l radicals, we and our co-workers have suggested (I, 3, 4) that this dependence of the experimentally observed +H formyl-proton couplings with the nature of the (X-) groups arises from the accompanying changes in the preferred radical conformations. In addition, the conformational preferences influence the formyl-proton coupling according to the empirical W-plan rule (6). Recent experimental EPR studies similar to those previously reported (I, 2) have yielded measurements of y-CH formyl-proton couplings of 2.24 and 2.20 G, respectively, for two further cY-formyloxy radicals, cH(CH,NH,)OCHO, 2, and cH(CH,OH)OCHO, 3, generated from vinyl formate at approximately 25°C in aqueous solution (7). As expected, the sizes of these experimental formyl-proton couplings are comparable to those for type-l radicals with no more than a single a-substituent (such as cH,OCHO, 4, 2.51 G;
all correspondence

should

be addressed. 265 0022-2364/82/080265-07$02.00/O Copyright 0 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.

266

KARUKSTIS

AND

SMITH

The present paper successfully extends the previous (I, 3, 4) INDO calculations to 2 and 3 in order to verify the earlier conclusions concerning the conformational dependence of the T-CH formyl-proton couplings of type-l radicals. Calculational studies of this type which assign preferred radical conformations on the basis of experimental results are not extensive (l-4,8), and thus this investigation is useful in clarifying the interpretation of long-range couplings in flexible aliphatic radicals. PROCEDURES

Culculational method. We applied the INDO-MO-SCF approximation as previously described (I, 3, 4). We once again emphasize the deficiency of this method in energy evaluations (8, 9) and make energy comparisons with caution. Geometries. As recommended for type-l radicals previously (1, 3, 4), we employed standard molecular bond lengths and angles (10) for the structural parameters. Although the extent of delocalization of the unpaired electron from the CP onto the /?-carbon is uncertain, our earlier calculations on type-l radicals using (C,-OB)-bond lengths corresponding to trigonal and tetrahedral radical centers resulted in similar qualitative conclusions (1, 3, 4). Accordingly, as before (4) the results presented are those obtained assuming sp* hybridization at C,. The prohibitively large size of 2 and 3 and the potentially unreliable energy results obtainable (9) rule out geometry optimization through energy minimization. We specified the bond-rotational angles considered as before (1, 3, 4),

I

the values of 0 and 4 defining (C,-0,) and (0,-C,) rotation, respectively. This specification applies to 2 and 3, provided we take the left-hand a-H in structure I to be the Lu-alkyl substituent. In structure III, 0 = 0” and Cp= O”, and in IV, 8 = 90” and $J = 0”:

III

IV

We consider the arrangement about the (0,-C,) bond to be s-tram in III and IV; C, and the formyl-hydrogen are oriented in an s-cis arrangement when 4 = 180”. As in the preceding investigations of type-l radicals (I, 3, 4), we carried

RADICALS

FROM

90

FIG. 1. Variation of the INDO-calculated a function of the angle of bond rotation

about

VINYL

FORMATE

267

0

y-CH formyl-proton C,-0, and O&J,,

coupling constant of 2, in gauss, respectively, 0 and Q, in degrees.

as

out simultaneous rotations about these bonds in increments of 10” for 0 and 20” for 4, to obtain a wide variety of possible rotational isomers for the radicals. In order to simplify the analysis of the dependence of the calculated formyl-proton coupling, a,, on 0 and 4, we fixed the entire (-NH,) group in 2 to be in the plane defined by the C of the (-C,H,NH,) group and the axis of the 2p, orbital at C,. Likewise, we arranged both atoms of the (-OH) group in 3 to be in the plane defined by the C of the (-C,H,OH) group and the axis of the C, 2p, orbital. The sizes of the P-CH2NH2 and -CH20H proton couplings in 2 and 3, respectively (7), support these choices. Although the (-NH,) group in 2 is likely protonated under the conditions employed for radical generation (I, 2, II), i.e., =CH(CH,NH,+)OCHO, 2, previous studies (4) showed that the choice of structure, 2 or 2’, will not affect the conclusions of the present paper. Hence, we considered only 2. RESULTS

Formyl-Proton

Coupling-Constant

AND

DISCUSSION

Values: General Trends

Figure 1 gives for 2 a plot of a, vs 6 and 0 contoured with respect to 8 and 4. The dependence of a, on 13and 4 for 2 is similar to that found for 3 and to that reported for the previously studied singly a-substituted type-l radicals (3, 4), with a, varying smoothly with B and 4, reaching maximum, positive values near conformations in the W-plan arrangement, B = 0” and 4 = O”, and falling to small and negative values with rotations about the (C,-0,) bond, 8, and the (0,-C,) bond, 4. The three general trends relating conformation with a, found for type-l radicals (4) apply to 2 and 3: (1) for a given radical, to maintain the same a, value, an increase in 0 from O”, the W-plan arrangement, to 90”, the planar conformation, demands that the range of allowed @Jvalues contract toward the s-trans conformation; (2) along the series 4 to 7, 2, 3, 8, for a particular 0 value, similar or

268

KARUKSTIS

AND

SMITH

120

60

180

6”

FIG. 2. Variation of the INDO-calculated a function of 19and +, in degrees; see Fig. proton coupling-constant values.

-y-CH formyl-proton coupling constant of 2, in gauss, as 1. The contour lines are labeled with respect to y-CH formyl-

increased allowed +value ranges about the s-trans conformation are necessary to obtain the same a, values; and (3) in the series 4 to 7, 2, 3, 8, for a given 4 value, similar or increased allowed O-value ranges about the W-plan arrangement are necessary to obtain equivalently sized long-range couplings. Figure 2 illustrates for 2 the dependence of a, on 0 and r$ contoured with respect to a,. This plot closely resembles that found for 3 and those reported earlier for other singly a-substituted type-l radicals (4). Such plots are particularly useful in recognizing the aforementioned trends.

180

FIG. 3. Variation of the INDO-calculated of the angle of bond rotation about Cm-O8

relative and O&,,

energy of 2, in kilocalories per mol as a function respectively, B and 6, in degrees.

RADICALS

FROM

VINYL

TABLE

1

CALCULATEDENERGYDIFFERENCESANDBARRIERS Energy

0 60 120 180

269

FORMATE

values

(kcal

FORROTATIONABOUTC,-Oa mol-‘pb

4

5

6

7

2

3

8

0.72 0.73 -1.10 1.10 -1.07 1.25 -0.79 0.79

0.46 0.80 -1.40 1.40 -1.47 1.67 -1.25 1.25

0.24 0.73 -1.53 1.53 -1.69 1.90 -1.41 1.41

0.41 0.85 -1.21 1.21 -1.51 1.72 -1.24 1.24

0.27 0.63 -1.65 1.65 -1.74 1.91 -1.33 1.33

-0.37 0.42 -1.35 2.13 -2.08 2.44 -1.34 1.94

6.17 11.21 -1.29 1.29 -2.83 2.85 2.02 3.02

* The data for 4 to 8 are from earlier reports (3, 4). bThe upper number of each pair is the energy difference, [E(0 = 90’) - E(B = O“)]. The lower number is the energy barrier, IAIZ(B&,,., the absolute value of the energy difference between the maximum and minimum E values for conformations about 0.

Energy Values for (Cm-O,)-Bond

Rotations

The plots of the INDO-calculated relative energy, E, vs B and 4 for radicals 2 and 3 are very similar to those found previously for the singly a-substituted type1 radicals 4 to 7 (3, 4). Figure 3 shows the plot for 2. Table 1 gives the energy differences, [E(B = 90”) - E(B = OO)], and barriers, ]AE(0)lmax, for rotation about C,-OB as a function of 4 for 2 and 3. As before (3, 4), in every case, we found free rotation to occur about the (Q-0,) bond. Energy Values for (O&,)-Bond

Rotations

Table 2 presents for 2 and 3 the energy differences, [E(4 = 180”) - E(@ = OO)], rotation as a function of 8. As found and barriers, ]AE(~)lmax, for (O&,)-bond TABLE CALCULATED

ENERGYDIFFERENCESANDBARRIERSFORROTATIONABOUTO&, Energy

0 30 60 90

2

values

(kcal

mol-‘Pb

4

5

6

7

2

3

8

0.19 1.45 -0.05 1.73 -0.80 2.16 -1.32 2.39

0.51 1.34 0.21 1.63 -0.62 2.05 -1.19 2.33

0.47 1.35 0.24 1.63 -0.61 2.08 -1.18 2.34

0.41 1.24 0.19 1.72 -0.66 1.21 -1.23 0.85

0.43 1.14 0.19 1.66 -0.62 2.09 -1.17 2.34

0.81 0.90 0.42 1.54 -1.07 2.49 -0.16 1.78

0.72 1.02 -0.35 1.85 -6.62 12.71 -3.42 9.97

’ As in Table b As in Table

1, footnote 1, footnote

(1. 6.

270

KARUKSTIS

AND TABLE

CALCULATED Upper limit of 4 (“1 0 60 120 180

SMITH 3

VALUES Average

OF (q) a, values

(G)‘@

4

5

6

7

2

3

8

1.95 2.86 1.10 0.80 0.73 -0.09 0.36 -0.18

1.91 2.53 1.36 0.47 0.83 -0.13 0.38 -0.17

1.93 2.38 1.40 0.56 0.85 -0.14 0.39 -0.18

2.00 2.64 1.48 0.86 0.92 -0.07 0.44 -0.13

1.98 2.59 1.47 0.84 0.92 -0.08 0.45 -0.13

1 .I5 1.68 1.30 0.06 0.80 -0.15 0.34 -0.19

2.87 4.16 1.68 1.24 1.07 -0.17 0.63 -0.17

o Calculated assuming rotation about C,-O,, 0 = = 0’ to the cited 4 value. The upper number of each mean value assuming free rotation over the specified i.e., the arithmetic mean value taking into account 25’C. b As in Table 1, footnote a.

O-180”, and with partial rotation about 0,-C,, Q pair is the simple average, i.e., the simple arithmetic limits. The lower number is the Boltzmann average, the E values and assuming the temperature to be

for type-l radicals 4 to 7, (3, 4), these differences and barriers depend on 0 in very values suggest free rotation about the (0,-C,) similar fashions. The laE(4)(,,, bond, although, as a result of known limitations of the INDO method discussed earlier (8, 9), such values may be underestimated. Assignment

of Preferred

Conformations

Previous consideration (3, 4) of the preferred conformations of cu-formyloxy radicals using the values of E and a, in conjunction with the experimental EPR data was most successful in predicting trends in preferred conformations with the assumption that the Ia,1 values were consistent with the experimental couplings in a relative manner rather than in an exact way. As shown earlier in Table 1, the IAJwaIaxvalues support free rotation about C,-0, for all fixed values of 6. Nevertheless, simple averaging of a, for (C,-O,)-bond rotation with similar fixed ranges of 4 for each radical results in a trend in the average value of a,, (a?), for the series 4 to 7, 2, 3, 8 opposite to that for the experimental couplings; see Table 3. The Boltzmann-averaging method, which takes into account the E data, gives the same trend as the simple-averaging procedure. However, based on our earlier work (4), we would expect free rotation about C,-OB, with an increased tendency for (0,-C,) rotation down the series of radicals 4 to 7, 2, 3, 8, to bring both the simple and Boltzmann averages of a, into line with the experimental results. Such is the case; see Table 3. Thus, the present calculations on 2 and 3 support the previously formulated (3, 4) conclusions of free rotation about the (G-0,) bond for type-l radicals, with enhanced (s-tram)-to-(s-h) isomerization following from increased a-substitution by bulkier groups.

RADICALS

FROM

VINYL

FORMATE

271

REFERENCES I. 2. 3. 4. 5.

6. 7. 8. 9.

10. II.

P. P. P. P. (a)

SMITH, R. A. KABA, L. M. D~MINGUEZ, AND S. M. DENNING, J. Phys. Chem. 81, 162 (1977). SMITH AND K. K. KARUKSTIS, J. Mugn. Reson. 39, 137 (1980). SMITH, K. K. KARUKSTIS, AND S. M. DENNING, J. Mugn. Reson. 40, 91 (1980). SMITH AND K. K. KARUKSTIS, J. Mugn. Reson. 42, 208 (1981). J. A. POPLE, D. L. BEVERIDGE, AND P. A. DOBOSH, J. Chem. Phys. 47, 2026 (1967); (b) J. A. POPLE, D. L. BEVERIDGE, AND P. A. DOBOSH, J. Am. Chem. Sot. 90, 4201 (1968); (c) J. A. POPLEANDD. L. BEVERIDGE, “Approximate Molecular Orbital Theory,” Chap. 4, McGrawHill, New York, 1970. (a) G. A. RUSSELL AND K.-Y. CHANG, J. Am. Chem. Sot. 87,438l (1965); (b) G. A. RUSSELL, K.-Y. CHANG, AND C. W. JEFFORD, J. Am. Chem. Sot. 87,4383 (1965). Lurs M. DOMINGUEZ, Ph.D. dissertation, Duke University, Durham, North Carolina, 1979. F. W. KING, Chem. Rev. 76, 157 (1976). (a) J. A. POPLE AND G. A. SEGAL, J. Chem. Phys. 44, 3289 (1966); (b) M. FROIMOWITZ AND P. J. GANS, J. Am. Chem. Sot. 94, 8020 (1972); (c) A. K. Q. SIU AND E. F. HAYES, Chem. Phys. Lett. 21, 573 (1973); (d) W. R. WADT AND W. A. GODDARD III, J. Am. Chem. Sot. 96, 1689 (1974); (e) A. G. GREGORY ANDM. N. PADDON-ROW, J. Am. Chem. Sot. 98,752l (1976); (f) A. VEILLARD, in “Quantum Mechanics of Molecular Conformations” (B. Pullman, Ed.), Chap. 1, Wiley, London, 1976; (g) J. I. FERNANDEZ ALONZO, in “Quantum Mechanics of Molecular Conformations” (B. Pullman, Ed.), Chap. 2, Wiley, London, 1976; (h) T. A. HALGREN, D. A. KLEIER, J. H. HALL, JR., L. D. BROWN, AND W. N. LIPSCOMB, J. Am. Chem. Sot. 100, 6595 (1978). J. A. POPLE AND M. GORWN, J. Am. Chem. Sot. 89,4253 (1967). (a) H. FISCHER, Z. Nuturforsch. A 19, 866 (1964); (b) W. T. DIXON, R. 0. C. NORMAN, AND A. L. BULEY, J. Chem. Sot., 3625 (1964); (c) C. CORVAJA, H. FISCHER, AND G. GIACOMETTI, Z. Phys. Chem. (Frankfurt am Main) 45, 1 (1965); (d) J. DEWING, G. F. LONGSTER, J. MYA’IT, AND P. F. TODD, Chem. Commun., 391 (1965); (e) H. TANIGUCHI, K. FUKUI, S. OHNISHI, H. HATANO, H. HASEGAWA, AND T. MARUYAMA, J. Phys. Chem. 72, 1926 (1968).