Chemistry 4361/8361 Wednesday, October 17

In-Class Exercise: Multiple-Bond Correlations in HMBC

The 1H, 13C, 1H-13C HMQC and 1H-13C HMBC spectra shown on the following pages are of -terpinene. -Terpinene is achiral, and the central ring is nearly planar. As a result, all six H8 & H9 protons are equivalent, as are C8 and C9; the two H5 protons are nearly equivalent; and the two H6 protons are nearly equivalent. Assign all of the resonances in the 1H and 13C spectrum to a nucleus in -terpinene, by filling in the chart below:

1H resonances:

13C resonances:

(ppm) Name(s) of

hydrogen(s) (ppm)

Name(s) of carbon(s)

5.65 142.3

5.61 132.9

2.31 119.5

2.14 116.4

2.11 34.4

1.79 28.9

1.05 25.2

22.8

21.1

10 4

5 6

3 2

1 7

8

9

-terpinene

Some notes on the attached spectra:

In the HMQC spectrum, the 13C pulse frequency was set to be centered about ~80 ppm. Using a complex Fourier transform (including imaginary terms, rather than only real ones) makes it possible to obtain data on 13C nuclei both above and below this frequency. However, the complex FT sometimes generates mirrored peaks on both sides of the frequency center. Usually—as in this case—it is fairly easy to determine which peaks are real cross-peaks, and which are “ghost” peaks.

2D NMR methods sometimes have baseline trouble with resonances of vastly different intensities, and this is illustrated in the full HMBC spectrum. The tall methyl group resonances generate false, baseline peaks at many 13C frequencies; these can be eliminated by setting the cutoff “floor” high above these peaks. Unfortunately, when you do that, crosspeaks from weak resonances (like those on the left-hand side of the spectrum) disappear. To address this issue, the close-up spectra have floors set appropriate to their region of the spectrum. As a result, I think you will find the HMBC close-ups more useful than the full HMBC spectrum.

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5Chemical Shift (ppm)

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

Nor

mal

ized

Inte

nsity

5.83.04.11.11.01.0

1.04

881.

0623

1.79

45

2.35 2.30 2.25 2.20 2.15 2.10 2.05

2.07

922.09

062.

1058

2.11

882.13

082.

1438

2.15

952.

1709

2.26

902.28

252.

29562.30

912.

3227

2.33

622.

3493

5.65 5.60

5.60

735.

6183

5.64

335.

6459

5.65

435.

6569

1H NMR, 500 MHz, in CDCl3

144 136 128 120 112 104 96 88 80 72 64 56 48 40 32 24 16Chemical Shift (ppm)

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

Nor

mal

ized

Inte

nsity

21.1

107

22.8

208

25.2

148

28.9

269

34.4

407

116.

3894

119.

5009

132.

9226

142.

2570

13C NMR, 125 MHz, in CDCl3

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0F2 Chemical Shift (ppm)

24

32

40

48

56

64

72

80

88

96

104

112

120

128

136

144

F1 C

hem

ical

Shi

ft (p

pm)

mirrored peaks,due to FT error

mirrored peak,due to FT error

1H-13C HMQC, 500/125 MHz, in CDCl3

(closeup 1)

(closeup 2)

5.80 5.75 5.70 5.65 5.60 5.55 5.50 5.45F2 Chemical Shift (ppm)

112

114

116

118

120

122

124

126

128

130

132

134

136

138

140

142

144

F1 C

hem

ical

Shi

ft (p

pm)

1H-13C HMQC, 500/125 MHz, in CDCl3(closeup 1)

2.0 1.5 1.0F2 Chemical Shift (ppm)

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

F1 C

hem

ical

Shi

ft (p

pm)

1H-13C HMQC, 500/125 MHz, in CDCl3(closeup 2)

5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0F2 Chemical Shift (ppm)

24

32

40

48

56

64

72

80

88

96

104

112

120

128

136

144

F1 C

hem

ical

Shi

ft (p

pm)

(closeup 2)

(closeup 3)(closeup 1)

1H-13C HMBC, 500/125 MHz, in CDCl3

(closeup 4)

5.80 5.75 5.70 5.65 5.60 5.55 5.50 5.45F2 Chemical Shift (ppm)

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

F1 C

hem

ical

Shi

ft (p

pm)

1H-13C HMBC, 500/125 MHz, in CDCl3(closeup 1)

5.75 5.70 5.65 5.60 5.55 5.50 5.45F2 Chemical Shift (ppm)

114

116

118

120

122

124

126

128

130

132

134

136

138

140

142

144

F1 C

hem

ical

Shi

ft (p

pm)

1H-13C HMBC, 500/125 MHz, in CDCl3(closeup 2)

2.0 1.5 1.0F2 Chemical Shift (ppm)

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

F1 C

hem

ical

Shi

ft (p

pm)

1H-13C HMBC, 500/125 MHz, in CDCl3(closeup 3)

2.0 1.5 1.0F2 Chemical Shift (ppm)

112

114

116

118

120

122

124

126

128

130

132

134

136

138

140

142

144

146

F1 C

hem

ical

Shi

ft (p

pm)

1H-13C HMBC, 500/125 MHz, in CDCl3(closeup 4)