A graph theory approach to structure solution of network materials from two-dimensional solid-state NMR data

An NMR crystallography strategy is presented for solving the structures of materials such aszeolites and related network materials from a combination of the unit cell and space group information derived from a diffraction experiment and a single two-dimensional NMR correlationspectrum that probes nearest-neighbour interactions.

 By requiring only a single 2D NMR spectrum, this strategy overcomes two limitations of previous approaches. First, the structures of materials having poor signal-to-noise in solid-state NMR experiments can be investigated using this approach since a series of 2D spectra is not required. Secondly, 

the structures of aluminophosphate materials can potentially be determined from ²⁷Al/³¹P solid-state NMRexperiments since this approach does not require the isolated spin pairs which have been important for determining structures of silicate materials by ²⁹Si solid-state NMR. Using concepts from graph theory, the structure solution strategy is described in detail using a hypothetical two-dimensional network structure. A collection of two-dimensional network structures generated by the algorithm under various initial conditions is presented.

 The algorithm was tested on a series of 27 zeolite framework types found in the International Zeolite Association’s zeolite structure database. Finally, the structure of the zeolite ITQ-4 was solved from powder X-ray diffraction data and a single ²⁹Si double quantum NMR correlation spectrum. The limitations of the strategy are discussed and new directions for this approach are outlined.

CrystEngComm, 2013,15, 8748-8762

DOI: 10.1039/C3CE41058G

http://pubs.rsc.org/en/content/articlelanding/2013/ce/c3ce41058g#!divAbstract

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Ha: Split by Hb (J geminal = 0.5-3 Hz from above) and by Hc (J trans = 12 -18 Hz from above). Because they’re different we apply the compound N+1 rule and get a doublet of doublets.

Hb: Split by Ha (J trans = 12-18 Hz) and Hc (J cis = 7-12 Hz). Of the three It looks most like a doublet of doublets since it has the largest J values (peaks are more spaced out)

Hc: Split by Ha (J geminal = 0.5-3 Hz) and Hb (J cis = 7-12 Hz). Also adoublet of doublets, and very similar to Ha except the peaks are closer since J cis < J trans.

13C NMR

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Compound below has a composition a 68.2% C, 13.6% H and 18.2% O by mass.

• A notable feature of the ¹H spectrum is a multiplet of nine at d1.7. What is the cause of this multiplet?

A splitting pattern of nine peaks must be produced by eight adjacent protons (8+1)

¹H Spectrum

¹H Spectrum (expansion)

• Sketch the possible isomers of compound U, eliminating any which do not account for this multiplet of nine.

• Assign each of the peaks, giving reasons for the relative intensities and splitting patterns.

Peak  is due to protons in position d as it has a chemical shift of d3.6 (corresponds to a -OH containing functional group) and is a triplet (split by two adjacent hydrogens - H_c).
Peak  is due the proton in position b as it has a chemical shift of d1.7 (corresponds to a tertiary -CH- group) and is a nontet (split by eight adjacent hydrogens - H_a and H_c).
Peak  is due to protons in position c as it has a chemical shift of d1.4 (corresponds to a -CH₂ group) and is a quartet (split by three adjacent hydrogens - H_b and H_d).
Peak  is due to protons in position e as it has a chemical shift of d1.3 (corresponds to hydroxyl proton) and is a singlet (no splitting).
Peak  is due the proton in position a as it has a chemical shift of d0.8 (corresponds to a -CH₃ group) and is a doublet (split by one adjacent hydrogens - H_b).

¹³C Spectrum

• The ¹³C spectrum shows the reference peak and four peaks due to compound U. Explain.

Peak  is due to the carbon atom in position d as it has a chemical shift of d60.7 (corresponds to a -C-O- functional group) and is connected to an even number of protons (2).
Peak  is due to the carbon atom in position c as it has a chemical shift of d41.5 (corresponds to a secondary carbon group) and is connected to an even number of protons (2).
Peak  is due to the carbon atom in position b as it has a chemical shift of d24.6 (corresponds to a tertiary carbon group) and is connected to an odd number of protons (1).
Peak  is due to the two carbon atoms in position a as it has a chemical shift of d22.5 (corresponds to a primary carbon group). It can be seen that the carbon atoms are also connected to an odd number of protons (3).

3,4-dihydroxyphenylacetaldehyde (DOPAL)

Scheme 2.1: Pinacol-pinacolone rearrangement of epinephrine to 3,4-dihydroxyphenylacetaldehyde.


3,4-dihydroxyphenylacetaldehyde (DOPAL), 
which is not commercially available. We originally set out to synthesize the compound based on a paper by Kim (Kim 1996) that proposed the rearrangement in one step using perchloric acid in glacial acetic acid. Due to the risks associated with perchloric acid (can be explosive when heated), we sought out other means of synthesizing DOPAL. 
In 1966, Robbins (Robbins 1966) synthesized a sodium bisulfite addition of DOPAL from epinephrine (adrenaline) using a phosphoric acid solution. Robbins based much of his work on a previously published paper by J. H. Fellman for which he proposed a pinacol-pinacolone type mechanism for the rearrangement of epinephrine (14) to DOPAL (15) in phosphoric acid (Fellman 1958) (Scheme 2.1). 
Using the same procedure (Robbins 1966), we attempted to synthesize DOPAL (15) from commercially available epinephrine (adrenaline) (14).

Spiro[indeno[1,2-b]pyrrole-3,2'-pyrroles]

Compounds 3 are red crystal substances readily soluble in DMSO and DMF, poorly soluble in other common organic solvents, and insoluble in saturated hydrocarbons and water. The products give a positive test (cherry-red coloration) with iron(III) chloride for the presence of enol hydroxyl groups. The IR spectra of 3 have absorption bands inherent to stretching vibrations of the enolic hydroxy group (3161–3188 cm−1, broadened band), two lactam carbonyl groups C5'=O (1763–1781 cm−1) and C2=O (1715–1732 cm−1), and two ketone carbonyl moieties C4=O (1667–1680 cm−1) and C3'-C=O (1640–1647 cm−1). 

1H-NMR spectra of 3 display signals of protons in the aromatic rings and substituents attached thereto, two doublets from the protons of the ethylene fragment of the cinnamoyl substituent (δ 7.64–7.66 and 7.72–7.75 ppm) with coupling constant (3J) values of about 16 Hz, and a broadened singlet from the enolic hydroxy proton (δ 13.31–13.45 ppm). In the 13C-NMR spectra of 3d we have observed carbon atom signals of the aromatic and aliphatic fragments, the carbonyl carbon atom of the cinnamoyl moiety (δ 182.46 ppm), ketone carbonyl carbon atom C4 (δ 183.74 ppm), lactam carbonyl carbon atoms C2 (δ 171.89 ppm) and C5' (δ 165.38 ppm), and spiro carbon atom (δ 68.34 ppm). 


The structure of 3c was unambiguously confirmed by single-crystal X-ray crystallography (Figure 1 and Figure 2). 






3'-Cinnamoyl-4'-hydroxy-1,1'-di(4-methylphenyl)-1H-spiro[indeno[1,2-b]pyrrole-3,2'-pyrrole]-2,4,5'(1'H)-trione (3c): Red solid, yield 79%, mp 282–283 °C. IR νmax: 3174 (OH), 1763 (C2=O), 1727 (C5'=O), 1678 (C4=O), 1647 (C3'-C=O) cm−1. 1H-NMR δ: 2.28 (s, 3H, Me), 2.43 (s, 3H, Me), 6.39 (d, 1H, H-8, J = 7.3 Hz), 7.01–7.69 (m, 16H, H-arom), 7.64 (d, 1H, COCH=CHPh, J = 15.8 Hz), 7.74 (d, 1H, COCH=CHPh, J = 15.8 Hz), 13.31 (s, 1H, OH).

Molecules 2012, 17(12), 13787-13794; doi:10.3390/molecules171213787


http://www.mdpi.com/1420-3049/17/12/13787/htm