Multidimensional NMR Studies on Polyurethane ... - ACS Publications

Chapter 11

Downloaded by PENNSYLVANIA STATE UNIV on September 9, 2012 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0834.ch011

Multidimensional NMR Studies on PolyurethaneBased Dendritic Wedges 1

2,

3

Minghui Chai , Peter L. Rinaldi *, Uraiwan Puapaiboon , and RichardT.Taylor 3

1

Department of Chemistry, Marshall University, Huntington, WV 25755-2520 Department of Chemistry, Knight Chemical Laboratory, The University of Akron, Akron, OH 44325-3601 Departments of Chemistry and Biochemistry, Miami University, Oxford, OH 45056

2

3

Multidimensional NMR techniques with pulse field gradient (PFG) enhancement have been applied to study 1 , 2 and 3 generation polyurethane dendritic wedges, which are a novel type of linking element in the convergent approach to dendrimer synthesis. By using PFG- HMQC and HMBC 2D NMR experiments, H and C resonance assignments have been obtained for these compounds, and structure proofs have been accomplished. st

1

nd

rd

13

© 2003 American Chemical Society

In NMR Spectroscopy of Polymers in Solution and in the Solid State; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

147

148


Introduction Dendrimers, which are highly symmetrical cascade polymers, have drawn a lot of attention because of their unique physical and chemical properties such as low intrinsic viscosity, high solubility, high miscibility, and high reactivity. The co-existing features, extensive branching and high surface functionality, have distinguished dendrimers from the classic linear and cross-linked polymers. So it is very important to know detailed information about the structure of dendrimers in order to understand their special properties and to devise new applications for these compounds. NMR has been a very powerful tool for structural characterization. However, until now, NMR has not been the preferred method for studying dendrimers, despite the wealth of information available from measurable NMR parameters. This is largely a result of the limited ability to resolve the resonances from the many unique, but very similar groups in these molecules. Because of the high cost and the difficulty of preparing isotopically labeled polymer and dendrimer samples, multidimensional NMR methods used in biomolecular structure determination have not been used to study dendrimers. To date, ID NMR has been used as a routine method for characterization of dendrimers. Only a few multidimensional NMR studies on dendrimers have been reported.(J, 2,3) Polyurethane dendritic wedges (Scheme 1) are a novel type of linking element in the convergent approach to dendrimer synthesis. By using multidimensional PFG-HMQC and HMBC experiments, confirmation of the *H and C resonance assignments have been obtained for these compounds, and structure proofs have been accomplished. 1 3

Experimental

NMR Measurements st

nd

rd

About 150 mg of each sample (1 , 2 and 3 generation wedges^ was dissolved in 0.7mL deuterated acetone in a 5 mm NMR tube for NMR studies. NMR spectra were obtained on a Varian Umtyplus 750 MHz NMR spectrometer equipped with four RF channels, a Performa II ζ axis pulse field gradient (PFG) accessory, and a Varian W C/ P/ H four channel triple resonance probe with a PFG coil. Acetone-de was also used as internal references for both H (2.05 ppm) and C (29.92 ppm) chemical shifts. All experiments were performed at 25.0 ± 0.1°C. All data were processed with Varian's VNMR software on a SUN Ultra-10 workstation. l

]

l3

3}

2

1 3


149

Scheme 1 st


1 Generation Wedge

2

nd

Generation Wedge

2 3

rd

Generation Wedge

3


150 ID NMR Experiments ]

The H spectra of all generation wedges were acquired at 750 MHz using a 3.5 s acquisition time, 8 kHz spectral width, 2.8 ps (30°) pulse width and 16 transients. All C spectra were acquired at 188.6 MHz using a 0.8 s acquisition time, a 20 kHz spectral width, an 8.0 ps (45°) pulse width and averaging of 4800 transients with WALTZ-16 modulated *H decoupling.


1 3

2D-NMR Experiments X

3

Y? C gradient HMQCft) spectra of all generation wedges were collected with a relaxation delay of 1 s, Δ = (2 χ VJ/C)" = 3.57 ms (optimized for 1 bond H - C correlations), and a 0.05 s acquisition time. The gradient strengths of three 2.0 ms PFGs were 0.462, 0.462, and -0.235 Tm" , respectively. H C gradient H M B C ^ spectra of all generation wedges were collected with a relaxation delay of 1 s, a 0.256 s acquisition time, a delay, Δ = (2 χ V/yc)" = 3.57 ms for suppressing coherences from 1-bond correlations and a delay, τ = (2 χ J )' = 0.05 ms for optimizing crosspeak intensities from long-range correlations. The gradient strengths of two 2.0 ms pulse field gradients (PFGs) were 0.462 and 0.342 Tm" , respectively. For both 2D experiments, U and C 90° pulses of 8.8 and 13.0 με respectively, were used; 8000 Hz and 32000 Hz spectral windows were used for the U (f ) and C (/}) chemical shift dimensions, respectively; 16 transients were averaged for each of 1024 real ti increments. 2D data were processed with sinebell weighting; zero filling was used so that 2D FT was performed on an 8192 χ 8192 matrix; and the spectra were displayed in the magnitude-mode in both dimensions. 1

]

13

1

!

1 3

1

n

]

HC

1

X

X

1 3

1 3

2

Results and Discussion X

The U NMR spectrum of 1 (Figure 1) shows resolved resonances for all the protons in the structure except for some of the aliphatic methylene protons in the long aliphatic chain. However, the C NMR spectrum (Figure 2) displays nicely resolved resonances for each carbon in the structure, including those in the long aliphatic chain. Therefore by using PFG HMQC and HMBC 2D NMR experiments, the proton resonances can be dispersed based on the resolved carbon resonances. In Figure lc, the singlet at 8.51 ppm is attributed to the amide proton. The singlets at 7.27 and 6.97 ppm are from H4 and H /H6 of the benzene ring, respectively. The singlet around 2.82 ppm is the resonance from the water in 1 3

2



151

' I' ·

1

'

I' ·* * I** · · ι

4.15

4.06

*

*

1

* t

ι ι tι ι ι . . . , tt . i , *

a 85

3.95

3.65

3.75

ppm

r

solve it ^ 1 1

I

21

1 1

'' I ''' I 1

A .

1 1

'' I '' " I '' ' I ''' ' I '' 'I ' • I ' 1

1.9

1.7

1

1

1.5

1

1

1

1:3

JJL

'I

1

'

1 1

I ' ''I ' 1

1.1

J

1 1

0.9

'1 "

1 1

I '

1 1 1

I

1

ppm

L I

6

1

1

1

1

3

I

2

ppm

Figure 1. 750 MHz *H ID NMR spectrum and the partial structure 1: (a) the expansion of alkoxy region, (b) the expansion of the aliphatic region, and (c) the full *HNMR spectrum.

the sample. Assignments of some of the resonances in the expansion shown in Figure la can be made based on the relative peak intensities, multiplicities and their chemical shifts; the triplet at 4.10 ppm is attributed to H . This triplet splitting is from coupling with two neighboring H - atoms. The triplet at 4.06 ppm is from the H atoms, which are coupled with two H atoms; and the doublet of triplet at 3.73 ppm is from the H atoms, which are coupled to the OH proton and two Hg protons. Here JH-M. h is 5.25 Hz and JHM is 6.00 Hz. The small triplet at 3.59 ppm is from the - O H proton, which is coupled with two H protons. The positions of the resonances from both - O H and >NH protons are concentration dependent. In Figure lb, the quintet near 2.05 ppm is from the residual protons of deuterated acetone solvent. The quintet around 1.95 ppm is attributed to the Hg protons, which are coupled with two H protons and two H r

2

7

8

9

0

9

7


9

152 protons. A second quintet near 1.64 ppm is from the Η · protons, which are split by two H protons and two H . protons. Because the two coupling constants in each multiplet are very similar (ca. 7.5 Hz), they each look like a quintet instead of triplet of triplets. The resonances in the 1.30 - 1.50 ppm region are attributed to the H - - H? protons in the dodecyl chains. Among them, two quintets near 1.39 ppm and 1.34 ppm are resolvable. However, based on the ID *H spectrum it is not possible to assign them. The triplet at 0.88 ppm is from the methyl groups (Hi ) in the dodecyl chains, which are coupled to two nearby H methylene protons. Figure 2 shows the 188.6 MHz C ID NMR spectrum of 1. In addition to the septet from the solvent that is marked by *s, nearly all twenty carbon resonances can be resolved. However, without the aid of 2D-NMR, only partial assignments can be made. For example, in the downfield region, the signal at 141.63 ppm can be assigned to C and C of the benzene ring based on the chemical shifts and relative peak intensities. But for the peaks at 154.52 ppm and 161.01 ppm, it is not possible to determine which is from Q of the benzene ring and which is from the >C=0 carbons. The peaks at 101.75 and 100.10 ppm can be assigned to C and C2/C6 of the phenyl ring, respectively based on their relative intensities. In the alkoxy region (Figure 2a), Three resonances at 65.60 ppm, 59.17 ppm and 65.32 ppm are well resolved, however it is hard to make specific assignment. In Figures 2b and 2c, the highest field resonances at 14.44, 23.41, and 32.72 ppm can be attributed to Ci ., C and Qos respectively, based on the known chemical shifts of similar carbons in longchain hydrocarbons. However it is not possible to assign the remaining methylene carbon resonances based on the ID spectrum alone. Thus 2D HMQC and HMBC NMR experiments were performed to complete the resonance assignments. 2

r

3

3

2


i r

1 3

3

5

4

2

i r

c )|ΙΙΙΙ|ΙΙΙΙ|ΙΠΙ|ΙΙΙΙ|ΜΙΙ|Ι1ΙΐρΐΙ1|Π1

66 64 62 ppm

11111|ΙΙΐμ

l|nil|lll|liqilll|ll |Π1ψΙΐρ»ΐρΐ

33.5 33.0

ΙΐηΐΙΠ|!ΙΙψΙΙ1[1ΠΙ|ΙΠΙ|ΙΙΙΙ|ΙΠΙ|ΙΙΙΙ|ΙΙΠ|ΙΙΠ|ΙΙΙΙ|ΙΙΙ

ppm

30.4

30.1

29.8 ppm

a

i 110

11 I I I 1111 11 11 I 111 111111111 111111111 1111 Ν 11

90

80 70 60 50 40 30

ppm

1B

Figure 2. J 88.6 MHz C ID NMR spectrum of 1 with expansions of alkoxy (a) and aliphatic (b and c) regions ; the solvent (acetone-dç) resonances are marked with *.


153 The 2D HMQC and 2D HMBC spectra of 1 are shown in Figures 3 and 4, respectively. The resonance assignments of the crosspeaks have been marked in both spectra according to the numbering in the partial structure shown in Figure 1. The ^ - ^ C HMQC crosspeaks from atoms 4 and 2/6 of the benzene ring are shown in Figure 3a. Figure 3b displays clear ^ - ^ C 1-bond correlations from the - O C H - groups. The ' Η - 0 HMQC correlations from aliphatic region are shown in Figure 3c. All the U- C HMQC crosspeaks are well resolved except methylenes 4' through 9'. Based on the H resonance assignments (discussed above) and the correlations observed in Figure 3b, it is possible to assign the - O C H - C resonances as labeled in the figure. Similarly the C resonance assignments of C»c

12'

2 , 6

c = o

1

1

δ'Η F2

\·

1

: /

2 , 6 ^

2,6

:

! — 4 1

1 1 1 1 1• 1

170

!

1"

160

ι

1

1

1 1 1 1

150

1

1

1

1 1 \

V

ι ι ι ι ι ι ι ι ι ι ι ι ι ι Γ l i t

140 130 F1 (ppm)

120

110

1

100

13

Figure 4. H C PFG-HMBC 2D NMR spectrum ofl: (a) and (b) the expansions of alkoxy regions, (c) and (d) the expansions of aliphatic regions, and (e) the downfield (aromatic and carbonyl) regions.

obtain many of the assignments labeled in Figure 3. It can also be used to verify the assignments of *H and C resonances obtained from the ID *H, C and 2D HMQC experiments discussed above. Figures 4a-d show the long-range ^ - ^ C (HMBC) correlations of 1 in the upfield (aliphatic and alkoxy) regions. The crosspeaks from - O C H - groups can be clearly assigned based on the additional long-range K- C correlations. For instance, C is correlated with both Hs (2-bond) and H (3-bond); C is only correlated with H (2-bond). Thus l 3

1 3

2

X

3

7

9

r

2


155 these resonances can be distinguished. In Figure 4b, C9 exhibits correlations with the OH, Hg and H protons. Thus it is easy to differentiate H / C , C i / C and H 9 / C 9 from the HMBC experiment. In a similar way the Hg/C , H 3 / C 3 ' , Ηιο'/Cio·, Hn'/Cir and H ./Ci - resonance assignments can be confirmed. In Figure 4c, the 2 and 4' methylene carbon resonances both exhibit long-range correlations with the H and Η · resonances, enabling their resolution and confirming the H /C - and I V C ^ resonance assignments. Furthermore, based on the H4/C4' assignments, Hy/C? resonances can be assigned. The Hpresonance can be assigned based on its HMBC crosspeak with do-. Figure 5 shows the expansions of the methylene (4-9') regions from the HMBC spectrum plotted with different thresholds. In Figure 5a, the crosspeak between C > and H . confirms the assignment of 2' methylene group. Similarly the crosspeaks between C . and H4. and between C » and H ., which are labeled in Figure 5b, provide information for the assignments of 4' and 5' methylene groups. However, due to the limited resolution in the HMBC spectrum, the resonances from the 6', Τ and 8' methylene groups are not resolved. Figure 4e shows the ^ - ^ C correlations of 1 in the downfield region of the HMBC spectrum. The resonance from d exhibits correlations with the H and H /H6 resonances; the resonances from C3/C5 are correlated with the resonances from Ht and H /H6; and the carbonyl resonance is correlated with H , providing a proof of the carbonyl resonance assignment. The C resonance has a correlation with the H /H6 resonance while the C / C resonance is correlated with the H4 resonance. These correlations, which are clearly shown in the HMBC spectrum, prove the assignments discussed previously. The *H and C resonance assignments of 1 are summarized in the Table 1. Figures 6, 7, 8 and 9 illustrate the ID *H NMR, ID C NMR, 2D HMQC and 2D HMBC spectra of 2. Figures 10, 11, 12 and 13 illustrate the ID C NMR, ID *H NMR, 2D HMQC and 2D HMBC spectra of 3. For both 2 and 3, the resonances from dodecyl chains are very similar to the ones in the spectra of 1. Therefore the assignments for these resonances can be concluded based on the assignments of 1. The major differences between the spectra of 1, 2 and 3 are exhibited in the resonances of amide, aromatic and alkoxy groups. With increasing generation, the NMR spectra become more complicated in these regions. However, it is still possible to distinguish most of the resonances of higher generation structures in the ID and 2D NMR spectra. Here, the 1 generation dendritic wedge (1) has been used to demonstrate the NMR characterization of these compounds. In the same manner, the 2 and 3 generation dendritic wedges (2 and 3) have been characterized; chemical shift assignments for these compounds are summarized in Table 1. 7

7

7

r

8

12

2

1

r


2

2

3

2

3

3

4

5

7

2

2

r

4

2

2

6

1 3

1 3

1 3

st

nd

rd



l

n

Figure 5. The expansions of aliphatic regions from the H CPFG-HMBC 2D NMR spectrum of 1: (a) the 2D plot with lower threshold (2), and φ) the 2D plot with higher threshold ( 3); both plots are at the same vertical scales.



157

l

nd

Figure 6. 750 MHz H ID NMR spectrum and the partial 2 structure of 2: (a) the expansion of the aromatic region, (b) the expansion of the alkoxy region, (c) the expansion of the aliphatic region, and (d) the full *HNMR spectrum.



306

η

156

304

152

30J?

148

300

144

29 8

ppm

296

102.5

101.0

π—I

λ 65 64

1 ppm

L

JLi

62 61

J

130 13

110 90

80

70

60

50

40

30

ppm

Figure 7. 188.6 MHz C ID NMR spectrum of 2 with expansions of crowded regions: a through d shown in the insets; the solvent (acetone-da) resonances are marked with *.

150

ι ι 11 l ι I 11 I I I 11 11111 1111111111 11 111 Μ 11 ι 11 1111111 i ι ι 11 11 Η ι Μ ι I j I ι ι 111 ι ι ι j ι ι ι 111 ι

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3 6 3 4 3 2 3 0 2 8 2 6 2 4 2 2 2 0 F1(ppm)

18

16

14

tfrgwre 8. *H C PFG-HMQC 2D NMR spectrum of 2: (a) the expansion of the aromatic region, (b) the expansion of the -OCH -region, and (c) the expansion of the aliphatic region. 13

2



160

IT | l l II | IIJ l | 1111| III11 III l | II H [ 1111| 11 II | II11| 11M11II111 II

664 6*4 61 57 34 F1(ppn) Flfem)

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160

150

I «' ' 140

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120

F/gwre P. / f C PFG-HMBC 2D NMR spectrum of 2: (a and b) expansions of the alkoxy regions, (c and d) expansions of the aliphatic regions, and (e) the expansion of the downfield (aromatic and carbonyl) regions. 7

7 3



161

Figure 10. 750 MHz *HNMR spectrum of 3: (a) expansion of the amide region, (b) expansion of the aromatic region, (c) expansion of the alkoxy region, (d) expansion of the aliphatic region, and (e) the full Ή NMR spectrum.


162

1

1

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S3

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ρμη

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u ι » 1 1 1 1 1 u 11 π u ; H I I [ I I I | j I II t j I I M j I I I I j M 111 M I t j 11 I I j 111 n I I I l | I I I t j 1111111

150

130

110

90

80

70

60

50

40

30

ppm

13

Figure 11. 188.6 MHz C ID NMR spectrum of 3 with expansions of crowded regions: a through d shown in the insets; the solvent (acetone-d

%

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1 2.6 3.5 4 7 8 9 OH ΝΗ(Γ) C=0 (2") 10 11 12 13 14,18 15,17 16 ΝΗ (l ) C=0 (2 ) 19 20 21 22 23,27 24,26 25 ΝΗΟ^ 0=0(2"") Γ 2' 3' 4' 5' 6' Τ 8' 9' 10' 11' 12'

6.97 7.27 4.06 1.95 3.73 3.59 8.51

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Multidimensional NMR Studies on Polyurethane ... - ACS Publications

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