- Email: [email protected]
Long-range deuterium-induced isotope effects on 1H chemical shifts in cis- and trans-stilbene
Journal
of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 4 IO-41 1 ( 1997) 9- I?
Long-range deuterium-induced isotope effects on ‘H chemical shifts in cis- and trans-stilbene P. Novak”, Z. Me%“‘*, D. VikiC-TopiCa, H. Sterkb aDepartment of Chemistry, Rudder BoSkoviC Institufe, PO Box 1016, IOOOIZagreb, Croatia ‘Department of Organic Chemistry, Karl-Franzens University, 8101 Graz, Austria
Received 26 August 1996; accepted 19 September 1996
Abstract Deuterium isotope effects on olefinic proton chemical shifts in a series of deuterated cis- and rrans-stilbene isotopomers have been determined and analysed. The effects exhibit conformational and solvent dependence. The sign and magnitude of the effects are governed by the additivity rule. An opposite trend between the effects at the olefinic proton and those previously observed at olefinic carbon has been found and discussed for closely related compounds. 0 1997 Elsevier Science B.V. Keywords:
Deuterium isotope effects;
‘H NMR; cis-Stilbene;
tmns-Stilbene;
Conformation
1. Introduction
2. Experimental
Isotope effects on 13C and ‘H NMR chemical shifts have proven to be a sensitive probe of molecular conformation and geometry in solution [l-4]. Deuterium-induced isotope effects on 13C chemical shifts have frequently been studied, while this is not the case with the effects on ‘H shifts. Long-range effects, that is those transmitted over more than three bonds from the site of the isotopic substitution, have rarely been observed in ‘H NMR [5]. We have recently determined long-range deuterium isotope effects on both ‘H and 13C chemical shifts in truns-N-benzylideneaniline isotopomers and found their usefulness in conformational studies [3]. In the present paper we investigate deuterium isotope effects on ‘H shifts in cis- (cSB) and truns-stilbene (tSB) (Fig. 1) and relate them to a-electron molecules of the same type.
‘H NMR spectra were recorded with Bruker AM360 and Varian Gemini 300 spectrometers, from acetone-& and chloroform-d, solutions in 5-mm NMR tubes at 294 K. Sample concentrations were 20-30 mg ml-‘. Deuterium from the solvent was used as the lock signal and TMS as the internal standard. Narrow region spectra with spectral widths of 5001500 Hz were zero filled to 64 K, thus giving a digital resolution better than 0.01 Hz per point. To determine precisely the sign, deuterium isotope effects were measured from mixtures of isotopically labelled and unlabelled species, prepared in different ratios. A positive sign denotes an upfield chemical shift. Experimental errors (Table 1) are obtained by summing standard deviations calculated for the chemical shifts of deuterated and undeuterated compounds. The preparation of deuterated isotopomers of cSB and tSB was described previously [6].
* Corresponding author.
0022-2860/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO22-2860(96)09679-2
10
P. Novak et al./Journal of Molecular Structure 410-411
1
separated. The shortest deuterium isotope effect possible in our systems is the one over three bonds, 3A, in 2 and 2’. “A in 2 is much smaller than in 2’ (Table 1) which is consistent with the trend observed in similar x-molecules, i.e. 3A-cis is smaller than 3A-trans [7]. The larger “A has been observed in chloroform-d1 than in acetone-d6 solution of 2’. It is worth noting that in o-‘H-styrenes the respective effects are significantly larger (3A-cis = 6.0 ppb, 3A-trans = 13.0 ppb) than here. Moreover, in linear *H-acetylene the value of even 16 ppb was measured for 3A [8]. The reduction of 3A in 2 and 2’ can be attributed to the influence of the second phenyl ring. The influence of the phenyl ring seems to be the opposite for deuterium isotope effects over one bond on “C-(Y chemical shifts in mentioned molecules. Namely, larger ‘A was observed at C-a in phenylacetylene than in acetylene [8]. The same was found in sp2 hybridized systems, i.e. ‘A in 2 (338.6 ppb) is larger than ‘A in *H-styrene (323.2 ppb, 2H and ‘H in the cis position), both being larger than ‘A in *Hethene (273.6 ppb). Obviously, a phenyl group increases ‘A at C-U and decreases 3A at H-CL It is assumed that ring current effects which give rise to increase in the anisotropy of shielding are the cause for such behaviour. The anisotropy effect is the same in magnitude for both ‘H and 13C shifts, but more significant for protons owing to the narrow ‘H shift range. In 3 and 3’ no effect over six bonds was determined at the olefinic protons. However, a small negative 6A with the value of - 0.88 ppb was observed in the
1
Fig. 1.Isotopomers 2zoI-‘H; 3=4-‘H;
of cis- and trans-stilbene and atom numbering: 2’=(u-‘H; 3’=4_*H; 4 = 4_2H,oI_2H; 5 =
*H&ing); 4’ = 4-*HP-*H; 5’ = ‘Hs(ring); 6=‘Hs(ring), w2H; 7= *H&ing),cJ-*H; 6’ = ‘HS(ring), cr-‘H; 7’=2HS(ring),a’-2H: CY-*H,4’-‘H; 9=*H,a(rings); 8’ = 8=*H,(ring), *Hj(ring),a-*H,4’-*H; 9’ = *H&rings); 10=*H,&ings),w2H; 10’= *H ,&ings),cr-*H.
3. Results and discussion Deuterium-induced isotope effects in ‘H NMR spectra of cSB and tSB can be determined accurately only for the olefinic protons, as seen in Table 1. The determination of the sign of the isotope effect is depicted in Fig. 2. The phenyl region of the proton spectra is highly overlapped which prevents the accurate determination of isotope effects. Furthermore, the signals of the olefinic protons in tSB (H-o&) fall very near to the para protons (H-4) in acetone-& giving rise to less precise determination of the effects. For that reason we remeasured all tSB isotopomers in chloroform-d1 (Table l), where these lines are well Table 1 Deuterium Molecule
isotope effects on H-cu chemical Solvent
shifts in isotopomers
(1997) 9-12
of cSB and tSB”
Nppb 2
3
4
5
6
7
8
9
10
cSB Acetone-d6
2.32 (0.06) 2’
1.I2 (0.03) 3’
4’
3.30 (0.03) 5’
2.63 (0.07)
6’
7’
8’
8.57 (0.35) 8.09 (0.17)
3.52 (0.21) 2.92 (0.10)
7.76 (0.25) 7.43 (0.10)
1.58 (0.08) 9’
10’
tSB Acetone-& Chloroform-d,
a Experimental
5.16 (0.61) 6.27 (0.05)
errors (standard deviations)
5.21 (0.18) 5.53 (0.15) are given in parentheses.
- 2.45 (0.29) - 1.76 (0.22)
4.67 (0.17) 4.77 (0.08)
P. Novak et al./Joumal of Molecular Structure 410-411
(1997) 9-12
‘A-
11
l.!% ppb
D
‘JHD
b)
Fig. 2. Narrow region 360 MHz spectrum of (a) 10 and (b) the 2:l mixture of 10 and 1.
isoelectronic para-*H-trans-N-benzylideneaniline, 4-*H-tBA [3]. In contrast to 4-*H-tBA (AX spin system), the olefinic parts of cSB and tSB constitute a typical ABX spin system where the chemical shifts of H-a and H-o’ differ only slightly and two lines cannot be resolved at 360 MHz. Deuterium effects in all others isotopomers of cSB and tSB (Table 1) are cumulative or total, ‘A, since more than one deuterium is present in the molecule. The trends found for monodeuterated isotopomers are also preserved for polydeuterated. The only negative (deshielding) ‘A was observed in decadeuterated 9’, amounting to -1.76 ppb. The additivity, which is a common feature of effects [9] generally holds for deuterated isotopomers of cSB and tSB. If one compares isotopomers 2,2’, 6, 6’, 7 and 7’, it can be concluded that the effect in isotopomers 5 and 5’ should be negative at H-Al and positive at H-cr’. However, no effect was measured either in 5 or in 5’, due to the above-mentioned overlapping. Furthermore, half of the effect measured in 9’ is - 0.88 ppb, while comparing the effects in 2’, 6’ and 7’ one obtains the value of + 0.89 ppb for the total
effect in 5’. This indicates a slight deviation from the additivity rule for isotopomer 5’. In isoelectronic tBA, where a heteroatom is present at the P-position from the benzylidene ring, the nonadditivity of isotope effects on both ‘H [3] and 13C chemical shifts [lo] are even more pronounced. This was accounted for by unequal rotamer distribution for the isotopomers as a consequence of the isotopic substitution [2,10]. Contrary to cSB and tSB all deuterium effects are negative at H-a! in tBA owing to the influence of the nitrogen lone pair electrons. From Table 1 it can also be seen that the isotope effects are slightly solvent dependent in isotopomers of tSB.
Acknowledgements This research is part of Project No. l-07-139 of the Ministry of Science and Technology of the Republic of Croatia. Part of this work was carried out at the Karl-Franzens Universitat in Graz (Austria) and one
12
P. Novak et al./Joumal of Molecular Structure 410-411
of us (P.N.) thanks the Austrian and Research for a fellowship.
Ministry
of Science
References [I] Z. MeiC, P. Novak, D. VikiC-TopiC and V. SmreEki, Magn. Reson. Chem., 34 (1996)36. [2] P. Novak, D. VikiC-TopiC, E. Gacs-Baitz and Z. MeiC, Magn. Reson. Chem., 34 (1996) 610.
[3] [4] [5] [6] [7] [S] [9] [lo]
(1997) 9-12
P. Novak, Ph.D. Thesis, University of Zagreb, 1995. P. VujaniC and Z. MeiC, I. Mol. Struct., 293 (1993) 163. P.E. Hansen, Prog. NMR Spectrosc., 20 (1988) 207. P. Novak, Z. MeiC and H. Sterk, J. Chem. Sot., Perkin Trans. 2 (1996) 2531. M.C. Baird, J. Magn. Reson., 14 (1974) 117. J.R. Wesener, D. Moskau and H. Gtinther, Tetrahedron Lett., 26 (1985) 1419. C.J. Jameson and H.-J. Osten, J. Chem. Phys., 81 (1984) 4293. V. SmreEki, D. VikiC-TopiC, Z. MeiC and P. Novak, Croat. Chem. Acta, 69 (1996) 1501.
of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 4 IO-41 1 ( 1997) 9- I?
Long-range deuterium-induced isotope effects on ‘H chemical shifts in cis- and trans-stilbene P. Novak”, Z. Me%“‘*, D. VikiC-TopiCa, H. Sterkb aDepartment of Chemistry, Rudder BoSkoviC Institufe, PO Box 1016, IOOOIZagreb, Croatia ‘Department of Organic Chemistry, Karl-Franzens University, 8101 Graz, Austria
Received 26 August 1996; accepted 19 September 1996
Abstract Deuterium isotope effects on olefinic proton chemical shifts in a series of deuterated cis- and rrans-stilbene isotopomers have been determined and analysed. The effects exhibit conformational and solvent dependence. The sign and magnitude of the effects are governed by the additivity rule. An opposite trend between the effects at the olefinic proton and those previously observed at olefinic carbon has been found and discussed for closely related compounds. 0 1997 Elsevier Science B.V. Keywords:
Deuterium isotope effects;
‘H NMR; cis-Stilbene;
tmns-Stilbene;
Conformation
1. Introduction
2. Experimental
Isotope effects on 13C and ‘H NMR chemical shifts have proven to be a sensitive probe of molecular conformation and geometry in solution [l-4]. Deuterium-induced isotope effects on 13C chemical shifts have frequently been studied, while this is not the case with the effects on ‘H shifts. Long-range effects, that is those transmitted over more than three bonds from the site of the isotopic substitution, have rarely been observed in ‘H NMR [5]. We have recently determined long-range deuterium isotope effects on both ‘H and 13C chemical shifts in truns-N-benzylideneaniline isotopomers and found their usefulness in conformational studies [3]. In the present paper we investigate deuterium isotope effects on ‘H shifts in cis- (cSB) and truns-stilbene (tSB) (Fig. 1) and relate them to a-electron molecules of the same type.
‘H NMR spectra were recorded with Bruker AM360 and Varian Gemini 300 spectrometers, from acetone-& and chloroform-d, solutions in 5-mm NMR tubes at 294 K. Sample concentrations were 20-30 mg ml-‘. Deuterium from the solvent was used as the lock signal and TMS as the internal standard. Narrow region spectra with spectral widths of 5001500 Hz were zero filled to 64 K, thus giving a digital resolution better than 0.01 Hz per point. To determine precisely the sign, deuterium isotope effects were measured from mixtures of isotopically labelled and unlabelled species, prepared in different ratios. A positive sign denotes an upfield chemical shift. Experimental errors (Table 1) are obtained by summing standard deviations calculated for the chemical shifts of deuterated and undeuterated compounds. The preparation of deuterated isotopomers of cSB and tSB was described previously [6].
* Corresponding author.
0022-2860/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO22-2860(96)09679-2
10
P. Novak et al./Journal of Molecular Structure 410-411
1
separated. The shortest deuterium isotope effect possible in our systems is the one over three bonds, 3A, in 2 and 2’. “A in 2 is much smaller than in 2’ (Table 1) which is consistent with the trend observed in similar x-molecules, i.e. 3A-cis is smaller than 3A-trans [7]. The larger “A has been observed in chloroform-d1 than in acetone-d6 solution of 2’. It is worth noting that in o-‘H-styrenes the respective effects are significantly larger (3A-cis = 6.0 ppb, 3A-trans = 13.0 ppb) than here. Moreover, in linear *H-acetylene the value of even 16 ppb was measured for 3A [8]. The reduction of 3A in 2 and 2’ can be attributed to the influence of the second phenyl ring. The influence of the phenyl ring seems to be the opposite for deuterium isotope effects over one bond on “C-(Y chemical shifts in mentioned molecules. Namely, larger ‘A was observed at C-a in phenylacetylene than in acetylene [8]. The same was found in sp2 hybridized systems, i.e. ‘A in 2 (338.6 ppb) is larger than ‘A in *H-styrene (323.2 ppb, 2H and ‘H in the cis position), both being larger than ‘A in *Hethene (273.6 ppb). Obviously, a phenyl group increases ‘A at C-U and decreases 3A at H-CL It is assumed that ring current effects which give rise to increase in the anisotropy of shielding are the cause for such behaviour. The anisotropy effect is the same in magnitude for both ‘H and 13C shifts, but more significant for protons owing to the narrow ‘H shift range. In 3 and 3’ no effect over six bonds was determined at the olefinic protons. However, a small negative 6A with the value of - 0.88 ppb was observed in the
1
Fig. 1.Isotopomers 2zoI-‘H; 3=4-‘H;
of cis- and trans-stilbene and atom numbering: 2’=(u-‘H; 3’=4_*H; 4 = 4_2H,oI_2H; 5 =
*H&ing); 4’ = 4-*HP-*H; 5’ = ‘Hs(ring); 6=‘Hs(ring), w2H; 7= *H&ing),cJ-*H; 6’ = ‘HS(ring), cr-‘H; 7’=2HS(ring),a’-2H: CY-*H,4’-‘H; 9=*H,a(rings); 8’ = 8=*H,(ring), *Hj(ring),a-*H,4’-*H; 9’ = *H&rings); 10=*H,&ings),w2H; 10’= *H ,&ings),cr-*H.
3. Results and discussion Deuterium-induced isotope effects in ‘H NMR spectra of cSB and tSB can be determined accurately only for the olefinic protons, as seen in Table 1. The determination of the sign of the isotope effect is depicted in Fig. 2. The phenyl region of the proton spectra is highly overlapped which prevents the accurate determination of isotope effects. Furthermore, the signals of the olefinic protons in tSB (H-o&) fall very near to the para protons (H-4) in acetone-& giving rise to less precise determination of the effects. For that reason we remeasured all tSB isotopomers in chloroform-d1 (Table l), where these lines are well Table 1 Deuterium Molecule
isotope effects on H-cu chemical Solvent
shifts in isotopomers
(1997) 9-12
of cSB and tSB”
Nppb 2
3
4
5
6
7
8
9
10
cSB Acetone-d6
2.32 (0.06) 2’
1.I2 (0.03) 3’
4’
3.30 (0.03) 5’
2.63 (0.07)
6’
7’
8’
8.57 (0.35) 8.09 (0.17)
3.52 (0.21) 2.92 (0.10)
7.76 (0.25) 7.43 (0.10)
1.58 (0.08) 9’
10’
tSB Acetone-& Chloroform-d,
a Experimental
5.16 (0.61) 6.27 (0.05)
errors (standard deviations)
5.21 (0.18) 5.53 (0.15) are given in parentheses.
- 2.45 (0.29) - 1.76 (0.22)
4.67 (0.17) 4.77 (0.08)
P. Novak et al./Joumal of Molecular Structure 410-411
(1997) 9-12
‘A-
11
l.!% ppb
D
‘JHD
b)
Fig. 2. Narrow region 360 MHz spectrum of (a) 10 and (b) the 2:l mixture of 10 and 1.
isoelectronic para-*H-trans-N-benzylideneaniline, 4-*H-tBA [3]. In contrast to 4-*H-tBA (AX spin system), the olefinic parts of cSB and tSB constitute a typical ABX spin system where the chemical shifts of H-a and H-o’ differ only slightly and two lines cannot be resolved at 360 MHz. Deuterium effects in all others isotopomers of cSB and tSB (Table 1) are cumulative or total, ‘A, since more than one deuterium is present in the molecule. The trends found for monodeuterated isotopomers are also preserved for polydeuterated. The only negative (deshielding) ‘A was observed in decadeuterated 9’, amounting to -1.76 ppb. The additivity, which is a common feature of effects [9] generally holds for deuterated isotopomers of cSB and tSB. If one compares isotopomers 2,2’, 6, 6’, 7 and 7’, it can be concluded that the effect in isotopomers 5 and 5’ should be negative at H-Al and positive at H-cr’. However, no effect was measured either in 5 or in 5’, due to the above-mentioned overlapping. Furthermore, half of the effect measured in 9’ is - 0.88 ppb, while comparing the effects in 2’, 6’ and 7’ one obtains the value of + 0.89 ppb for the total
effect in 5’. This indicates a slight deviation from the additivity rule for isotopomer 5’. In isoelectronic tBA, where a heteroatom is present at the P-position from the benzylidene ring, the nonadditivity of isotope effects on both ‘H [3] and 13C chemical shifts [lo] are even more pronounced. This was accounted for by unequal rotamer distribution for the isotopomers as a consequence of the isotopic substitution [2,10]. Contrary to cSB and tSB all deuterium effects are negative at H-a! in tBA owing to the influence of the nitrogen lone pair electrons. From Table 1 it can also be seen that the isotope effects are slightly solvent dependent in isotopomers of tSB.
Acknowledgements This research is part of Project No. l-07-139 of the Ministry of Science and Technology of the Republic of Croatia. Part of this work was carried out at the Karl-Franzens Universitat in Graz (Austria) and one
12
P. Novak et al./Joumal of Molecular Structure 410-411
of us (P.N.) thanks the Austrian and Research for a fellowship.
Ministry
of Science
References [I] Z. MeiC, P. Novak, D. VikiC-TopiC and V. SmreEki, Magn. Reson. Chem., 34 (1996)36. [2] P. Novak, D. VikiC-TopiC, E. Gacs-Baitz and Z. MeiC, Magn. Reson. Chem., 34 (1996) 610.
[3] [4] [5] [6] [7] [S] [9] [lo]
(1997) 9-12
P. Novak, Ph.D. Thesis, University of Zagreb, 1995. P. VujaniC and Z. MeiC, I. Mol. Struct., 293 (1993) 163. P.E. Hansen, Prog. NMR Spectrosc., 20 (1988) 207. P. Novak, Z. MeiC and H. Sterk, J. Chem. Sot., Perkin Trans. 2 (1996) 2531. M.C. Baird, J. Magn. Reson., 14 (1974) 117. J.R. Wesener, D. Moskau and H. Gtinther, Tetrahedron Lett., 26 (1985) 1419. C.J. Jameson and H.-J. Osten, J. Chem. Phys., 81 (1984) 4293. V. SmreEki, D. VikiC-TopiC, Z. MeiC and P. Novak, Croat. Chem. Acta, 69 (1996) 1501.