Article pubs.acs.org/JPCA
Structural Investigation of HCN Polymer Isotopomers by SolutionState Multidimensional NMR Chao He,† Guangxin Lin,†,⊥ Kathleen T. Upton,† Hiroshi Imanaka,†,‡ and Mark A. Smith*,†,‡,§ †
Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States Department of Planetary Science, Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, United States § Department of Chemistry, University of Houston, Houston, Texas 77204, United States ‡
S Supporting Information *
ABSTRACT: Hydrogen cyanide is considered as an important precursor to amino acids and nucleic acids, and its polymers could have profound implications on prebiotic chemistry. Several structures of HCN polymers are speculated, but these structures are disparate both chemically as well as structurally. Here, we employ solution-state NMR spectroscopy to investigate the structure of HCN polymers with 13C and 15 N isotopic enrichment. From the multinuclear and multidimensional NMR investigations, we identify some discrete structural units for the most concentrated small molecular components and suggest that the dominating polymers are polyimine chain-like structures, which are formed by base-catalyzed nucleophilic addition reactions.
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INTRODUCTION Hydrogen cyanide has been discussed as a precursor to amino acids and nucleic acids in early Earth1−3 and polymerizes readily under a variety of different conditions.4−6 Hydrogen cyanide has been detected throughout our galaxy7 and undoubtedly plays a part in the synthesis of the prolific cyanoacetylenes detected in planetary atmospheres and the interstellar medium.8−10 Hydrogen cyanide polymers could have profound implications for prebiotic chemistry. The structure of these polymers still remains uncertain, though it has been studied for many years by various methods,1−6,11−25 including chromatography, IR spectroscopy, UV spectroscopy, mass spectrometry, and nuclear magnetic resonance (NMR) spectroscopy. On the basis of these studies, several possible but disparate structures have been speculated. These structures and their possible formation routes are shown in Figure 1. The dimer (Figure 1a), trimer (Figure 1b), and tetramer (Figure 1c,d) could be formed from hydrogen cyanide through new carbon−carbon bond formation.11 The tetramer, diaminomaleonitrile (DAMN), has two isomers, cis-DAMN (Figure 1c) and trans-DAMN (Figure 1d). These oligomers could polymerize as subunits further.4−6,14−18 Trans-DAMN is suggested to form a trans ladder structure (Figure 1f),4 while through a similar mechanism cis-DAMN would yield the cis form (Figure 1h).14 The polymerization of trimer could produce polyaminomalenitrile (Figure 1i), which could react with HCN to yield polyamidines (Figure 1j), converted to polypeptides (Figure 1k) by hydrolysis.5,6,15−18 However, Ferris argued against the hypothesis for the polypeptide formation and believed that the tetramer, DAMN (Figure 1c,d), rather than dimers or trimers should be the direct precursor to HCN polymers.19 Adenine (Figure 1l), a pentamer of HCN and a © 2012 American Chemical Society
Figure 1. Pathways and structures proposed for HCN polymerization in previous studies.2−4,13−18,20,21,24
Received: February 17, 2012 Revised: April 24, 2012 Published: April 24, 2012 4751
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polymer samples were also prepared in the same way, using either ammonia or triethylamine (0.5 wt %) as the catalyst. High-resolution (M/ΔM > 106) laser desorption/ionization mass spectrometry of the solid HCN before dissolution shows less than 0.7 atom % oxygen incorporation in the organic material. For these reasons, we firmly believe that the polymerization process is occurring under essentially anhydrous conditions. The solid was dissolved in DMSO-d6 (Cambridge Isotope Laboratories; D, 99.9%) for NMR measurement. The solid dissolved in DMSO-d6 completely after 10 min at room temperature. Solution-State NMR Spectroscopy. The NMR experiments were carried out on Bruker DRX 500 and Varian INOVA 600 spectrometers. The Bruker DRX 500 spectrometer is equipped with a Bruker 5 mm BBO S2 probe with z-gradient and the Varian INOVA 600 spectrometer is equipped with an H, C, N triple-resonance cryogenic probe with z-gradient. We carried out one-dimensional (1D) 1H and 13C NMR experiments with different decoupling methods, two-dimensional 1H 13 C heteronuclear single quantum correlation (HSQC), 1H 15N HSQC, 13C 15N HSQC, 1H 13C heteronuclear multiple bond correlation (HMBC), 1H 1H correlation spectroscopy (COSY), a 1H 13C correlation spectrum with a 15N-filtering tripleresonance pulse sequence [1H → 15N → 13C(t1) → 15N → 1 H(t2)] and also a 1H 13C correlation spectrum with a 15Nfiltering triple-resonance pulse sequence {1H → 15N → 13Ca → [13Ca]13Cb(t1) → 13Ca → 15N → 1H(t2)}. The 1D 13C spectrum was acquired on the Bruker DRX 500 spectrometer due to its excellent 13C sensitivity (BBO 13C probe coil). All other experiments were achieved on the Varian INOVA 600 spectrometer.
component of RNA, DNA, and ATP, has also been detected in the aqueous solution products.20−23 The extended triazine structure, as two-dimensional sheets (Figure 1n), was first suggested by Minard et al.13 Herzfeld et al. recently proposed that these two-dimensional sheets (Figure 1n) are formed by the further addition of one-dimensional chains (Figure 1m), which are first built up by head-to-tail addition of HCN monomers.24 However, the predominant polymer structures that have been proposed are disparate both chemically as well as structurally. Further detailed structural investigations are required to better understand this complex material. NMR spectroscopy, as a powerful tool for analyzing the structures of complex molecules, has been used to study the HCN polymer mixtures. In 1981, Ferris et al. reported 1H and 13 C NMR spectra of HCN oligomers in DMSO-d6 solution, and the NMR results suggested there are carboxamide and urea groups in the HCN oligomers as a result of their aqueous generation.10 Matthews and co-workers carried out solid-state NMR experiments on base-catalyzed HCN polymerization and determined the formation of new 13C−15N bonds by doublecross-polarization studies on polymers synthesized from equimolar amounts of H13CN and HC15N.13,15,16 Umemoto et al. obtained the 1H and 13C NMR spectra of HCN polymers and observed the broad N−H proton peak at 8−9 ppm in the 1 H spectrum and broad −CN− carbon peak at 140−170 ppm in the 13C spectrum.14 Recently, Herzfeld et al. characterized HCN polymers by solid-state NMR at higher field and faster magic-angle-spinning speed and obtained 13C and 15N solid-state NMR spectra.24 They also did the 13C double-cross-polarization experiment of a sample made from a 1:1 mixture of H13CN and HC15N, which indicated carbon forms new bonds with nitrogen and thus is bonded to more than the one nitrogen. They proposed a one-dimensional chain (Figure 1m) and two-dimensional sheet (Figure 1n) structures, whose chemical shifts for 13C and 15N would be consistent with the spectra. These few NMR studies on HCN polymers only acquired one-dimensional 1H, 13C, or 15N spectra, which support different structure proposals. These studies have not provided a self-consistent structural picture of this important material and further work is required. In the present article, we investigate the solution-state NMR spectra of labeled anhydrous H13C15N polymers comprehensively. From the NMR spectra, we determine specific structures of dominating small molecules or fragments within the polymer. The NMR results indicate that the HCN predominately polymerizes to a one-dimensional chain structure (polyimine) in the presence of trace ammonia or triethylamine. We believe that the polymerization of HCN is a base-catalyzed nucleophilic addition reaction and that the one-dimensional chains (polyimine) grow by forming new carbon−carbon bonds.
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RESULTS AND DISCUSSION In this section, we describe the NMR result of the polyH13C15N; first, the one-dimensional 1H and 13C spectra with different decoupling methods, followed by the two-dimensional NMR spectra (HSQC, COSY, HMBC, and triple-resonance experiments). On the basis of the NMR result, we assign the resolved sharp peaks to a set of small molecules and the broad peaks to the polymer structures. Figure 2 shows the 1H spectrum of poly-H13C15N with 13C and 15N decoupling. The peak at 2.5 ppm is from the proton impurity in the DMSO and the peak at 3.3 ppm is from water. The water could be an impurity of the NMR solvent (DMSOd6) or may come from the air during sample transfer and
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EXPERIMENTAL METHODS Synthesis. Fully labeled poly-H13C15N was prepared from pure H13C15N by amine catalysis.15,26 The H13C15N was prepared by heating a mixture of labeled K13C15N (Cambridge Isotope Laboratories; 13C, 99%; 15N, 98%+) and sulfuric acid to 60 °C. The evolved gas was dried with anhydrous calcium chloride and then collected into a flask cooled by an ice bath. After the H13C15N was collected, anhydrous ammonia gas (∼0.5 wt %) was admitted into the flask. The flask was sealed and placed at room temperature. After 12 h, the H13C15N polymerizes to a black-brown solid. Naturally abundant HCN
Figure 2. Solution-state 1H NMR spectrum of poly-H13C15N with 13C and 15N decoupling. 4752
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Table 1. 13C−1H and 15N−1H Coupling Constants (J) of the Resolved Peaks in the 1H Spectraa
dissolution. There are two major broad peaks (1.5−3.5 ppm and 5.5−9.0 ppm) along with some small sharp peaks. As we know, the sharp peaks are from small molecules and broad peaks are from much larger polymers in the NMR spectra. The integrated intensity of the 1H spectrum shows that the sharp peaks comprise only a small portion of the HCN polymer sample (∼5%) and that the broad peaks occupy the majority. That means that, in the HCN polymers, small molecules are very few and that polymerization is extensive. Since our sample is 13C and 15N labeled, the 13C−1H and 15 N−1H couplings can be observed. We obtained the 1H spectra with different decoupling methods and identified the 15 N−1H and 13C−1H couplings by comparing these spectra. As shown in Figure 3, the peak at 4.54 ppm is a singlet in the 1H
1
1
H (ppm) 4.54 4.58 4.835 4.842 5.40 7.16 7.42 7.67 7.96 7.95
2
J or 3J (Hz)
J (Hz)
1
JNH = 81.1 JNH = 82.5 1 JNH = 83.2 1 JNH = 81.7 1 JNH = 87.0 1 JNH = 87.2 1 JNH = 89.1 1 JNH = 89.8 1 JNH = 90.2 1 JCH = 185.4
JCH JCH JCH JCH
1
= = = =
5.2, 4.6, 4.5, 4.5,
5.2 4.6 4.5 4.5
(t) (t) (t) (t)
JNH = 15.4
a1
J reflects one-bond coupling constant, while 2J/3J implies a two- or three-bond long-range coupling constant.
Figure 3. Expanson of the solution-state 1H NMR spectra of polyH13C15N with different decoupling methods: (a) with 13C and 15N decoupling, (b) with only 13C decoupling, and (c) without 13C or 15N decoupling.
Figure 4. Solution-state 13C NMR spectrum of poly-H13C15N with 1H decoupling.
spectrum of poly-H13C15N with 13C and 15N decoupling, and it splits to a doublet in the 1H spectrum of poly-H13C15N with only 13C decoupling, which indicates that the doublet splitting is from 15N−1H coupling. The coupling constant (JNH) is 81.1 Hz, which is consistent with one-bond 15N−1H coupling.27 This doublet in the 1H spectrum of poly-H13C15N with only 13 C decoupling becomes a doublet of triplets in the 1H spectrum of poly H13C15N without 13C or 15N decoupling, which shows that the triplet splitting is from 13C−1H coupling. The coupling constant (JCH) is 5.2 Hz, which demonstrates that this is not one-bond 13C−1H coupling but two- or three-bond long-range 13C−1H coupling.28a The couplings indicate that this proton is bonded to nitrogen-15 and couples to two nearly identical carbons two- or three-bonds away. In the same way, the couplings of the other resolved peaks are identified and listed in Table 1. There are only the three nuclei, protons, carbon-13, and nitrogen-15, in our sample, so the proton bonds to either carbon-13 or nitrogen-15. Using different decoupling methods in the 1H spectra shows that almost all of the protons in the small molecules are bonded to nitrogen-15 (H−N) except the proton at 7.95 ppm, which is bonded to carbon-13 (H−C). There are also some two- and three-bond long-range 13 C−1H or 15N−1H couplings for the protons at 4.54, 4.58, 4.835, 4.842, and 7.95 ppm, which indicate that these protons connect to carbons or nitrogens two or three bonds away.27,28a The 13C spectrum of poly-H13C15N with 1H decoupling is shown in Figure 4. The peak at 39.5 ppm is from the carbon in DMSO. There is a combination of broad peaks with some small resolved peaks in the 13C spectrum. In accordance with the 1H
spectrum, the 13C spectrum also shows that the polymers reflect the majority of the mass, and the small molecules only constitute a small portion of the HCN polymers. Considering the fact that there are only the three nuclei, protons, carbon-13, and nitrogen-15, in our sample, we believe the broad unresolved polymer resonance at 60−100 ppm is from carbon in the sp3 carbon−nitrogen single bond (C−N), the signal at 110−135 ppm is from carbon in the carbon−carbon double bond (CC) or the carbon−nitrogen triple bond (CN) functionality, and the strong featureless resonance at 140−190 ppm is from carbon in the carbon−nitrogen double bond (C N) functionality.28b,c Similar to the 1H spectrum, the 13C spectra with different decoupling methods (Figure S1, Supporting Information) are obtained, and some 15N−13C and 1H−13C couplings of the resolved peaks are identified and listed in Table 2. The coupling constants (JNC or JHC) show that they are one-bond couplings.27,28a All the carbons listed in Table 2 bond to nitrogen-15, while the carbon at 162.9 ppm Table 2. 15N−13C and 1H−13C One-Bond Coupling Constants (J) of the Resolved Peaks in the 13C Spectra 1
13
J (Hz)
C (ppm) 113.9 157.9 159.6 162.5 162.9
4753
1
JNC 1 JNC 1 JNC 1 JNC 1 JNC
= = = = =
16.8 20.2 18.5 13.1 13.5, 1JHC = 185.4
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Table 3. 1H and 15N Chemical Shifts of the Strong Crosspeaks in the 1H 15N HSQC Spectrum
additionally is bound to a proton. The chemical shifts and the coupling constants indicate that the carbon at 113.9 ppm is from the CN group, the peaks at 157.9, 159.6, and 162.5 ppm are from CN groups, and the peak at 162.9 ppm is from the H−CN group. The result of 1H and 13C one-dimensional decoupling experiments show that there is only one proton (7.95 ppm) coupling with carbon through one-bond coupling and one carbon (162.9 ppm) coupling with a proton through one-bond coupling. On the basis of the chemical shifts, they could be bonded to each other. In order to verify this thought, we conducted 1H 13C HSQC two-dimensional experiments. The protons and carbons bonded directly can give crosspeaks in the 1 H 13C HSQC spectrum. In the 1H 13C HSQC spectrum of poly-H13C15N (Figure S2, Supporting Information), there is only one obvious crosspeak at (7.95, 162.9 ppm), which means that the proton at 7.95 ppm bonds to the carbon at 162.9 ppm directly. This result confirms our thought. The few crosspeaks above the noise level in the 1H 13C HSQC spectrum also support the observation that there are very few protons bound to carbon at least in the small molecule distribution in the HCN polymers. The 1H 13C HSQC spectrum shows in the HCN polymers that few protons are bonded to carbon, so it is natural to look at the protons bonded to the nitrogen as demonstrated in Figure 5. The 1H 15N HSQC spectrum reveals a diverse range of N−H
1
H (ppm)
4.54 4.58 4.835 4.842 5.40 5.76 6.78 7.16, 7.42 7.67, 7.96
15
N (ppm) 43.82 45.22 47.54 59.58 73.55 69.48 77.47 110.20 100.04
conformational isomerization leads to a common (or near common) 15N chemical shift but significant change in proton environment. In the latter case, thermodynamics would require the energy difference to be small, leading to near equal proton intensities, yet not with too small of an exchange barrier as to lead to proton convergence or coalescence on the NMR time scale. In order to determine that they are NH2 or NH groups, we performed a 1H 15N HSQC experiment without 1H decoupling on 15N. The 1H−15N one-bond couplings of the strong crosspeaks in the 15N dimension can be identified by this experiment, and the number of the protons bound to the nitrogens can be determined by the coupling patterns. The 1H 15 N HSQC spectrum without 1H decoupling on 15N (Figure S4, Supporting Information) shows that the strong crosspeaks at (7.67, 100.04 ppm) and (7.96, 100.04 ppm) split to triplets in the 15N dimension. The coupling constants fit the 1H−15N one-bond coupling constants equal to those observed in onedimensional 1H spectra (Table 1). The triplets indicate that the nitrogen at 100.04 ppm bonds to two protons and that the crosspeaks are from NH2 functional group. The other strong crosspeaks at (4.54, 43.82), (4.58, 45.22), (4.835, 47.54), (4.842, 59.58), (5.40, 73.55), (5.76, 69.48), (6.78, 77.47), (7.16, 110.20), and (7.42, 110.20) ppm also split into triplets, and they also derive from NH2 functional groups. Since the strong crosspeaks are from NH2 groups, the protons at 7.16/7.42 ppm and the protons at 7.67/7.96 ppm can only be in asymmetric NH2 groups where strong hydrogen bonding prevents proton equivalency. From the 1H 15N HSQC spectrum without 1H decoupling on 15N, we determine that the strong crosspeaks derive from NH2 functional groups. However, we cannot resolve the 1 H−15N couplings for the crosspeaks in regions A and B (Figure 5) corresponding to the higher polymer broad peaks (1.5−3.5 ppm and 5.5−9.0 ppm) in the 1H spectrum. The number of protons bound to these nitrogens cannot be determined by coupling patterns, but these crosspeaks do correspond to N(H)x functionality. The 1H, 13C, and 15N in our sample are all NMR spin 1/2 nuclei, which lend themselves to direct triple-resonance experiments. The result of 1H 13C HSQC and 1H 15N HSQC experiments shows that, in our sample, most of the protons are bound directly to nitrogen rather than carbon, so we carry out a 15 N-filtering 1H 13C two-dimensional experiment using the 1 H−15N−13C triple-resonance pulse sequence (as shown in the embedded diagram in Figure 6) and obtain a 1H 13C twodimensional long-range couplings NMR spectrum (Figure 6). Every crosspeak in this spectrum comes directly from an H− N−C structure. The 1H 15N HSQC experiments reveal that the
Figure 5. Solution-state 1H 15N HSQC spectrum of poly-H13C15N (1H and 13C decoupling on 15N; 13C and 15N decoupling on 1H). (Chemical shift reference of 15N: δN (NH3) = 0 ppm.)
bonding. These crosspeaks distribute from 1.5 to 9 ppm in the 1 H dimension. The crosspeaks in region A and B correspond to the higher polymer broad peaks (1.5−3.5 ppm and 5.5−9.0 ppm) in the 1H spectrum. On the basis of the chemical shifts, we think the crosspeaks in region A are from H−N (sp3) groups, and the crosspeaks in region B could be from H−NR groups. The same spectrum as that in Figure 5 with high contour is shown in Figure S3, Supporting Information, which only shows the strong crosspeaks corresponding to the sharp peaks in the 1H spectrum. The detailed chemical shifts of the H−N groups in small molecules are shown in Table 3. We find that the protons at 7.16 and 7.42 ppm connect to the same nitrogen at 110.20 ppm and that the protons at 7.67 and 7.96 ppm also connect to the same nitrogen at 100.04 ppm. They could be associated with asymmetric NH2 functionality where strong hydrogen bonding prevents proton equivalency. Another explanation is that these signals belong to the same single proton bound to nitrogen in a ring or rigid chain where 4754
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Figure 6. Solution-state 1H 13C spectrum of poly-H13C15N with 15Nfiltering triple-resonance pulse sequence (1H and 15N decoupling on 13 C; 15N and 13C decoupling on 1H). The embedded diagram shows the triple-resonance coherence transfer, and the embedded spectrum is the expansion of region A (in the same contour).
Figure 7. Solution-state 1H 13C HMBC spectrum of poly-H13C15N (1H and 15N decoupling on 13C; 13C decoupling on 1H). The embedded spectra are the expansions of the crosspeaks at (4.54, 101.4) and (4.835, 132.3) ppm (in the same contour).
crosspeaks at (4.54, 101.4) and (4.835, 132.3) ppm are given to show the 13C−13C coupling clearly. The 1H and 13C chemical shifts of the crosspeaks in the 1H 13C HMBC spectrum and the 13 C−13C coupling constants for some carbons are listed in Table 5. The carbons at 119.2, 126.7, and 132.3 ppm represent
N−H groups at (4.54, 43.82), (4.58, 45.22), (4.835, 47.54), (4.842, 59.58), and (5.40, 73.55) are all NH2 functional groups. Combined with the spectrum in Figure 6, we know they are C− NH2 groups. The 13C chemical shifts of these C−NH2 groups can be known from the spectrum in Figure 6. The precise chemical shifts of 1H, 15N, and 13C in these C−NH2 groups are shown in Table 4. In addition, we observe that some peaks split
Table 5. 1H and 13C Chemical Shift of the Crosspeaks on the 1 H 13C HMBC Spectrum and the 13C−13C Coupling Constants for the Relevant Carbons
Table 4. Chemical Shifts of H−N−C Structures and the 13 C−13C Coupling Constants from the 1H 13C Spectrum with 15 N-Filtering Triple-Resonance Pulse Sequence (Chemical Shifts of the 15N Are from the 1H 15N HSQC Spectrum) 1
H (ppm) 4.54 4.58 4.835 4.842 5.40
15
N (ppm) 43.82 45.22 47.54 59.58 73.55
13
H (ppm)
4.54 4.58 4.835 5.40 7.16, 7.42 7.67, 7.96 7.95
1
JCC (Hz)
C (ppm) 132.3 119.2 126.7 142.2 159.6
1
1
JCC JCC 1 JCC 1 JCC 1
= = = =
1
JCC = 66.65(t) JCC = 67.48(t) 1 JCC = 66.26(t) 67.85 1
coupling 2
3
JCH or JCH
1
J artifact
13
1
JCC (Hz)
C (ppm) 101.4 126.7 119.1 159.6 162.9 162.5 162.9
1
JCC = 92.4, 66.65(dd) 1 JCC = 1JCC = 66.26(t) 1 JCC = 1JCC = 67.48(t)
couplings with two carbons, and the carbon at 142.2 ppm is coupled with one carbon as also seen in the 1H 13C spectrum with the 15N-filtering triple-resonance pulse sequence (Figure 6). On the basis of the chemical shifts and the coupling constants of these carbons,28a they are assigned to sp2 carbons, and the couplings are sp2−sp2 13C−13C one-bond couplings. The carbon at 101.4 ppm has two 13C−13C one-bond couplings, the constants of which are 92.4 and 66.61 Hz, respectively. The coupling constants28a reveal that the 92.4 Hz coupling is a sp−sp2 13C−13C coupling and that the 66.61 Hz coupling is a sp2−sp2 13C−13C coupling. The carbons at 159.6, 162.5, and 162.9 ppm have no 13C−13C coupling, implying that theses carbons are not bound to carbon. The 1H 15N HSQC experiments and the 1H 13C spectrum with the 15N-filtering triple-resonance pulse sequence (Figure 6) display four C−NH2 groups whose 1H, 15N, and 13C chemical shifts are (4.842, 59.58, 142.2 ppm), (4.58, 45.22, 119.2 ppm), (4.835, 47.54, 126.7 ppm), and (4.54, 43.82, 132.3 ppm), respectively (Table 4). The 1H 13C HMBC spectrum shows that the proton at 4.58 ppm has two- or three-bond longrange couplings with the carbons at 126.7 and 142.2 ppm. The proton at 4.58 ppm is in a C−NH2 group (4.58, 45.22, 119.2 ppm) and is two bonds away from carbon at 119.2 ppm, so the couplings between the proton at 4.58 ppm and the carbons at 126.7 and 142.2 ppm can only be three-bond long-range
to doublets or triplets in the 13C dimension. Because the 13C is 1 H and 15N decoupled, these couplings can only be 13C−13C couplings. The expansion of region A is given to show the 13 C−13C couplings clearly. The coupling constants indicate that they are 13C−13C one-bond couplings. These 13C−13C coupling constants are also listed in Table 4. The chemical shifts and the coupling constants of these carbons28a indicate that they are sp2 carbons. The carbons at 119.2, 126.7, and 132.3 ppm couple with two nearly identical carbons, so the C-NH2 groups can be extended to CC(C)−NH2 groups. The carbon at 142.2 ppm is coupled to one carbon and has no proton attached, so it could be in the CC(N)−NH2 group. The carbon at 159.6 ppm has no 13C−13C coupling and no proton attached, so it could be in the NC(N)−NH2 group. The 1H 13C HMBC spectrum of the poly-H13C15N is shown in Figure 7. The peaks in this spectrum represent the 1H and 13 C, which have two- or three-bond long-range couplings (the crosspeak at 7.95, 162.9 ppm is a one-bond artifact). As observed in Figure 6, the 13C−13C couplings are observed for the carbons at 101.4, 119.2, 126.7, 132.3, and 142.2 ppm in the vertical dimension, and the coupling constants indicate they are 13 C−13C one-bond couplings. The expansions (Figure 7) of the 4755
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couplings. In other words, the carbon at 119.1 ppm is bonded to the carbons at 126.7 and 142.2 ppm, and the carbons at 126.7 and 142.2 ppm are also in C−NH2 groups [(4.835, 47.54, 126.7 ppm) and (4.842, 59.58, 142.2 ppm)]. These three C− NH2 groups are linked by carbon−carbon bonds. The 1H 13C HMBC spectrum shows that the proton at 4.835 ppm has a three-bond long-range coupling with the carbon at 132.3 ppm, and the proton at 4.54 ppm has three-bond long-range couplings with the carbons at 126.7 and 101.4 ppm. We can finally link the five carbons, four of which are in C−NH2 structures (Figure 8a). The spectrum in Figure 6 reveals that
Figure 9. Expansion of solution-state 1H 1H COSY spectrum of polyH13C15N without 13C or 15N decoupling on 1H (we only show the region that has crosspeaks).
HSQC spectrum shows that the proton at 7.95 ppm is bonded to the carbon at 162.9 ppm (CH). So, the nitrogen at 110.20 ppm can be connected to the carbon at 162.9 ppm (NH2− CH). The crosspeaks between the protons 7.16 and 7.42 ppm and the carbon at 162.9 ppm on the 1H 13C HMBC spectrum confirm this structure. The 1H 13C HMBC spectrum also shows that the carbon at 162.9 ppm has no 13C−13C one-bond coupling, so it is not bonded to carbon. The chemical shift28b indicates that it could have a double-bond with nitrogen (Figure 8c). One of the protons at 7.16 and 7.42 ppm should form strong hydrogen bonding with a nitrogen atom, which prevents proton equivalency and leads to these two protons having different chemical shifts. The 1H 13C HMBC spectrum shows that the carbon at 162.9 ppm only has crosspeaks with the protons at 7.16 and 7.42 ppm, so there is no proton bonded to the second nitrogen. Considering that this reaction starts with H13C15N units, the second nitrogen could be bonded to carbon (Figure 8c). The protons at 7.67 and 7.96 ppm are chemically similar to the protons at 7.16 and 7.42 ppm in that they are also from asymmetric NH2. The 1H 13C HMBC spectrum shows that there are crosspeaks between the protons 7.67 and 7.96 ppm and the carbon at 162.5 ppm. We think that they are two-bond long-range couplings and that the structure is C−NH2. The 1H 13 C HMBC spectrum shows the carbon at 162.5 ppm to have no 13C−13C one-bond coupling, so it is not bonded to carbon but only to nitrogen. The chemical shift28b indicates that it could have one double-bond and one single-bond with nitrogens (Figure 8d). Since the reaction starts with H13C15N units, there should be many carbon−nitrogen bonds in the sample. From the above results, we have already found several carbon−nitrogen bonds. The 13C 15N HSQC experiment is carried out to see these 13 C−15N couplings clearly. However, it is very hard to get the crosspeak in the 13C 15N HSQC experiment due to the low sensitivity of 15N. Only two obvious crosspeaks in the 13C 15N HSQC spectrum (Figure S5, Supporting Information) are observed. The crosspeak at 113.9, 253.2 ppm with the coupling constant 16.8 Hz (from 13C one-dimensional spectra) could be due to cyano groups. The other crosspeak at 159.6, 73.55 ppm is from the structure as shown in Figure 8b. At this point, most of the sharp peaks in the 1H and 13C spectra have been assigned to specific small molecules or fragments (Figure 8). The residual peaks are broad, including the peaks at 1.5−3.5 and 5.5−9 ppm in the 1H spectrum
Figure 8. Small molecule and three fragments are constructed based on the NMR results, all the chemical shifts are color coded (red represents 1H, black 15N, and blue 13C).
the couplings between these carbons are sp2−sp2 13C−13C couplings, so each carbon should have a double-bond to others. The carbon at 101.4 ppm (Figure 8) has one sp2−sp2 13C−13C coupling (66.61 Hz) with the carbon at 132.3 ppm and another sp−sp2 13C−13C coupling (92.4 Hz) with an sp-carbon, which could be a cyano group, and the carbon at 101.4 ppm has no proton attached, so it should have a bond to nitrogen. The carbon at 142.2 ppm only has one sp2−sp2 13C−13C coupling, so it should also have another bond to nitrogen. If the carbons at 101.4 and 142.2 ppm bond to the same nitrogen, a stable 6ring structure (Figure 8a) can be formed. The 1H 15N HSQC spectra and the 1H 13C spectrum with the 15N-filtering triple-resonance pulse sequence demonstrate that the carbon at 159.6 ppm is also in a C−NH2 structure (5.40, 73.55, 159.6 ppm) and has no 13C−13C coupling, so this carbon could bond to nitrogen and the 13C−15N couplings have been observed in the 13C spectra. The chemical shift indicates that the carbon at 159.6 ppm could have one double-bond and one single-bond with nitrogens.28b,c The 1H 13C HMBC spectrum shows that only one proton (5.40 ppm) has a crosspeak with this carbon, so the other two nitrogens could have no proton attached and be bonded to carbon or nitrogen. Considering that this reaction starts with H13C15N units, the other two nitrogens could at least be bonded to one carbon. Finally, the fragment structure as shown in Figure 8b can be drawn. The 1H 1H COSY experiment has been performed, and the crosspeaks are only observed in the downfield region. Figure 9 shows expansion of the 1H 1H COSY spectrum. This spectrum shows that the proton at 7.16 ppm has a crosspeak with the proton at 7.42 ppm, and the proton at 7.67 ppm has a crosspeak with the proton at 7.96 ppm, which supports our above judgment that the protons at 7.16/7.42 ppm and the protons at 7.67/7.96 ppm are from asymmetric NH2 because one single proton distributed between isomeric structures cannot give crosspeaks on an 1H 1H COSY spectrum. These crosspeaks represent H−H two-bond coupling. The protons at 7.16/7.42 ppm also have crosspeaks with the proton at 7.95 ppm by a three-bond long-range coupling. The 1H 15N HSQC spectra reveal that the protons at 7.16 and 7.42 ppm connect to the same nitrogen at 110.20 ppm (NH2), and the 1H 13C 4756
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(Figure 2) and at 60−100 and 140−190 ppm in the 13C spectrum (Figure 4). These broad features are from much larger polymers, but the structure of these polymers remains elusive. As shown in Figure 1, some possible structures of the polymers have been proposed in previous studies.2−4,13−18,20,21,24 On the basis of the NMR spectra in this article, we can address which structures are consistent with our data. The 1H 13C and 1H 15N HSQC spectra show that almost all protons bond to nitrogen rather than carbon in both small and large molecules as well as polymers. We can therefore conclude that the peptide-like structures (Figure 1i,j,k), the onedimensional chains (Figure 1m), and two-dimensional sheets (Figure 1n) are not present in significant amounts in our sample. The protons in the structures (Figure 1e,f,g,h) polymerized from DAMN (a form of the HCN tetramer) bond to nitrogen. Moreover, the broad peak at 1.5−3.5 ppm in the 1H spectrum (Figure 2), the broad peak at 60−100 ppm in the 13C spectrum (Figure 4), and the crosspeak in region A in the 1H 15N HSQC spectrum (Figure 5) are in accordance with the structures (Figure 1e,f,g,h). So these structures could be in the polymer distribution. The integration result shows that the broad peak at 1.5−3.5 ppm comprises 13% of the intensity in the 1H spectrum (Figure 2), and the broad peak at 60−100 ppm comprises about 10% of the intensity in the 13C spectrum (Figure 4), which suggests that the structures (Figure 1e,f,g,h) are not the major components in the polymer distribution. The integration result shows that the broad peak at 5.5−9.0 ppm comprises 75% of the intensity in the 1H spectrum (Figure 2) and the broad peak at 140−190 ppm 79% of the intensity in the 13C spectrum (Figure 4). It is known that these protons bond to nitrogen rather than carbon from the 1H 13C HSQC and 1H 15N HSQC spectra. According to the chemical shifts, these protons most likely are bonded to the sp2-nitrogens, which have double bonds with carbons (CNH), i.e., imines, and this type of carbon is consistent with the high broad peak at 140−190 ppm in the 13C NMR spectrum. None of the proposed polymer structures contain these types of protons or carbons. Considering that the broad peaks are composed of a heterogeneous distribution of polymers, we propose a new structure, polyimine chain-like structures (Figure 10c,d), which are consistent with the bulk polymer signals in the spectra (the broad peak at 5.5−9.0 ppm in the 1H spectrum, the broad peak at 140−190 ppm in the 13C spectrum, and the crosspeak in region B in the 1H 15N HSQC spectrum). The integration results of the 1H and 13C spectra suggest that the polyimine
chain-like structures represent more than 75% of the HCN polymers. On the basis of this polymer structure, we can propose a possible mechanism for the formation of the polyimine chainlike structures (Figure 10c,d). First, one hydrogen cyanide reacts with ammonia to give a cyanide ion and ammonium. Nucleophilic addition of cyanide anion to the carbon of another hydrogen cyanide forms intermediate a. Intermediate a can obtain a proton ion from HCN and generate the dimer of hydrogen cyanide (reaction 1). It takes place again, and 2iminoacetimidoyl cyanide (Figure 10b, a trimer of hydrogen cyanide) can be produced from the dimer. Repeating nucleophilic addition then produces the one-dimensional polyimine chain-like structures (Figure 10c). The protons in the chains form hydrogen bonds with neighboring nitrogens (Figure 10d), making the chain structures more stable. The polyimine chain-like structures can give broad peaks at 5.5−9 ppm in the 1H NMR spectrum and broad peaks at 140−190 ppm in the 13C NMR spectrum. Moreover, all the protons bond to nitrogens except at the terminus. The polyimine chainlike structures are in accord with the NMR results. The integration results of the 1H and 13C spectra suggest that the polyimine chain-like structures represent more than 75% of the HCN polymers. As mentioned above, a small amount of polymers (Figure 1e,f,g,h) formed from the DAMN (a tetramer of HCN) may be present in the samples. They account for about 10% in the sample and could be the byproduct of the HCN polymerization. The main reaction yields the polyimine, and the side reaction yields the polymers (Figure 1e,f,g,h). We also propose a possible mechanism for the side reaction (Figure 11). In the presence of ammonia, three HCN molecules form 2iminoacetimidoyl cyanide (Figure 10b, a trimer of hydrogen cyanide) as shown in Figure 10. Next, the 2-iminoacetimidoyl nitrile is attacked by cyanide ion. Usually, the cyanide ion attacks at the nitrile carbon, producing the polyimine chain-like structures. However, a small quantity of cyanide ion attacks at the carbon in the carbon−nitrogen double bond at the other end of 2-iminoacetimidoyl cyanide, producing the 2-amino-2cyanoacetimidoyl cyanide (Figure 11a), which will transform to DAMN (trans and cis). The trans- and cis-DAMN further polymerize to the polymers (Figure 1e,f,g,h). These structures are in accord with the peaks at 1.5−3.5 ppm in the 1H spectra, the peaks at 60−100 ppm in 13C spectra and the region A in 1H 15 N HSQC spectrum. The formation of DAMN is minor under our conditions, so the amount of the polymers (Figure 1e,f,g,h) is small (∼10% of the sample) compared to the polyimine chain-like structures (Figure 10c,d; more than 75%). We have now assigned most of the peaks of the 1H and 13C spectra, including both sharp and broad peaks. The sharp peaks (∼5%) are from some small molecules and fragments as shown in Figure 8. The broad peaks are mainly from the polyimine chain-like structures (Figure 10c,d; more than 75%) along with a small amount of polymers (Figure 1e,f,g,h; ∼10%] derived from DAMN. Our NMR study indicates that the major component of HCN polymers is the polyimine chain-like structures (Figure 10c,d) by forming new carbon−carbon bonds between HCN monomers, rather than the one-dimensional chains (Figure 1m) and two-dimensional sheets (Figure 1n) by forming new carbon−nitrogen bonds between HCN monomers. Herzfeld et al. considered the formation of new 13C−15N bonds mainly based on the 13C double-cross-polarization result of the mixed-
Figure 10. Possible mechanism for the formation of the polyimine chain-like structures from HCN. 4757
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Figure 11. Possible mechanism for the side reaction of HCN polymerization in the presence of ammonia.
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label sample (1:1 mixture of H13CN and HC15N).24 However, it is noticed that the intensity of the double-cross-polarization 13 C NMR spectrum of the mixed-label sample is much lower than the cross-polarization 13C NMR spectrum of the full labeled sample even with twice the amount of scans (8192 vs 4480). Considering the natural abundance of 13C in HC15N and 15N in H13CN, there are approximately 1.1% original 13 C−15N bonds in HC15N and 0.4% original 13C−15N bonds in H13CN. Thus, the conclusion that most carbons form new bonds with nitrogen is questionable. The low intensity of the double-cross-polarization 13C NMR spectrum24 can only suggest that few carbons form new bonds with nitrogen at best. For our sample, the signals in the 1H 13C correlation spectrum with 15N-filtering triple-resonance pulse sequence {1H → 15N → 13Ca → [13Ca]13Cb(t1) → 13Ca → 15N → 1 H(t2)} (Figure S6, Supporting Information) confirm the formation of carbon−carbon bonds. Moreover, the protons at 6−8 ppm have correlation with the carbons at 150−170 ppm in Figure S6, Supporting Information (1H−15N−13Ca−13Cb experiment), supporting the polyimine chain-like structures (Figure 10c,d). In order to verify our proposed mechanism, we synthesized HCN polymers in the presence of triethylamine and obtained the 1H NMR spectrum of this sample on the Varian INOVA 600 spectrometer. The 1H NMR spectrum of this sample is exactly the same with that of the sample catalyzed by ammonia, which indicates that the polymerization process of HCN is the same whether the catalyst is ammonia or triethylamine. In the presence of a base, the nucleophilic addition reaction takes place in the HCN system, the HCN molecules link to each other by forming a new carbon−carbon bond and the polyimine chain-like structures (Figure 10c,d; more than 75%) finally form. The side reaction yields DAMN, which polymerizes to a small amount of other polymers (Figure 1e,f,g,h; ∼10%). The few small molecules (∼5%) are some small terminal byproduct in the polymerization, which cannot polymerize further.
ASSOCIATED CONTENT
S Supporting Information *
Solution-state 13C NMR spectra of poly-H13C15N with different decoupling methods, 1H 13C HSQC spectrum, 1H 15N HSQC spectra, 13C 15N HSQC spectrum, and 1H 13C correlation spectrum of 15N-filtering 1H−15N−13Ca−13Cb experiment of poly-H13C15N. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 713/743-3755. Fax: 713/743-8630. E-mail:
[email protected]. Present Address ⊥
SABIC Innovative Plastics, 1 Lexan Lane, Mount Vernon, Indiana 47620, United States.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge support of this work through NASA Exobiology Grant NNG05GO58G and the NASA Astrobiology Initiative, through JPL subcontract 1372177. We also recognize the assistance of Dr. Tamara Munsch in the very early portion of this study.
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CONCLUSIONS In the present article, we investigate the solution-state NMR spectra of labeled H13C15N polymers comprehensively. By analyzing the NMR spectra carefully, we determine some specific structures of small molecules or fragments within the polymer and suggest a new structure and formation mechanisms for the polymers. In the presence of a base such as ammonia or triethylamine, the HCN mainly polymerizes to the polyimine chain-like structures (Figure 10c,d). The polymerization of HCN is a base-catalyzed nucleophilic addition reaction, and the one-dimensional chains (polyimine) grow by forming new carbon−carbon bonds. A side reaction forms some DAMN, which polymerize to a small amount of polymers (Figure 1e,f,g,h). 4758
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