Article pubs.acs.org/JPCA
Interplay of Intermolecular and Intramolecular Hydrogen Bonds on Complex Formation: The 3‑Aminopropanol−Water van der Waals Complex Andrew S. Khalil,†,# Anne-Marie Kelterer,‡ and Richard J. Lavrich*,† †
Department of Chemistry and Biochemistry, College of Charleston, 66 George Street, Charleston, South Carolina 29424, United States ‡ Institute of Physical and Theoretical Chemistry, Graz University of Technology, NAWI Graz., Stremayrgasse 9/Z2, Graz, Austria S Supporting Information *
ABSTRACT: This combined experimental and theoretical study answers the question whether the intramolecular hydrogen-bond strength in amino alcohols is dependent on the ring size. For this purpose, the rotational spectrum of the 3-aminopropanol−H2O van der Waals complex was recorded using Fourier-transform microwave spectroscopy and fit to the rotational, quadrupole coupling, and centrifugal distortion constants of the Watson A-reduction Hamiltonian. The experimental results are consistent with an ab initio conformation calculated at the MP2/6-311++G(d,p) level that involves the lowest energy 3-aminopropanol monomer and consists of a hydrogen bonding network. The calculated global minimum ab initio complex however comprises a higher energy monomer conformation of 3-aminopropanol. Upon complex formation with water, the O−H····N intramolecular hydrogen bond and OCCN backbone conformation of the lower energy monomer remain unchanged, in contrast to 2-aminoethanol. This behavior is consistent with the increasing strength of the intramolecular hydrogen bond of linear amino alcohols as a function of increasing chain length.
1. INTRODUCTION Amino alcohols contain NH2 and OH functional groups both of which may serve as hydrogen bond donor or acceptor. Interaction of these groups to form intramolecular hydrogen bonds serve to stabilize the molecular conformations adopted by amino alcohol monomers. Two possible intramolecular hydrogen bonding motifs may occur in amino alcohols, NH····O (amine to alcohol) and OH····N (alcohol to amine). Despite the potential for the formation of these intramolecular hydrogen bonding motifs in amino alcohols, microwave and gas phase IR studies of 2-aminoethanol (2AE),1−3 3-aminopropanol (3AP),4−6 and 4-aminobutanol (4AB)7 show evidence of the OH····N intramolecular hydrogen bond only. Amino alcohol monomers can be characterized in terms of the “ring-like” structure formed by the atoms involved in the intramolecular hydrogen bond and the remaining backbone heavy atoms with five-, six-, and seven-membered rings forming for 2AE, 3AP, and 4AB, respectively. The strength of the intramolecular hydrogen bond in the monomer is believed to increase as a function of “ring” size, the result of larger rings providing more flexibility to the backbone and hence stronger intramolecular hydrogen bonds. This increase in strength of the OH····N intramolecular hydrogen bond with larger ring size has been inferred from the shift of the OH stretch to lower frequency in 3AP6 relative to 2AE.3 Additional support has been provided by ab initio calculations.8 Compared to that in 2AE, a decrease in the OH····N intramolecular bond of 11.2% and 18.6% and slight increase in the H−O covalent bond of 0.2% and 0.8% were calculated for 3AP and 4AB, respectively. © XXXX American Chemical Society
Condensed phase IR and Raman measurements of solution phase 2AE9 and 3AP10 also support the stronger intramolecular hydrogen bonding in the 3AP monomer relative to 2AE. The formation of intermolecular hydrogen bonds may compete with the intramolecular hydrogen bonds stabilizing the isolated amino alcohol monomer. For 3AP, spectral features arising from both intermolecular hydrogen bonds between dimers and free monomers with intact intramolecular hydrogen bonds were observed.10 No such features attributed to the free monomer of 2AE9 were observed indicating sacrifice of its intramolecular hydrogen bond in favor of intermolecular interactions between dimers. Formation of 1:1 van der Waals complexes of amino alcohols with water allows for the introduction of intermolecular hydrogen bonds. As with the amino alcohol monomer, water can serve as both a hydrogen-bond donor and acceptor. The formation of intermolecular hydrogen bonds has the potential to disrupt the intramolecular hydrogen bond found to stabilize conformations of amino alcohol monomers. The degree to which the hydrogen bond donor and acceptor sites in water may interfere with the intramolecular hydrogen bond in the amino alcohol monomer depends upon the strength of the intramolecular hydrogen bond and is likely a function of the size of the ring discussed above. As a result, one might expect a Received: June 12, 2017 Revised: August 5, 2017 Published: August 10, 2017 A
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The Journal of Physical Chemistry A decreasing degree of disruption upon formation of water complexes of 2AE, 3AP, and 4AB, respectively. A previous microwave study of the 2AE−H2O11 complex demonstrated that the OH····N intramolecular hydrogen bond in the 2AE monomer was sufficiently weak such that during complexation with water, it is sacrificed in favor of forming a intermolecular hydrogen bond with the water oxygen (as well as a intermolecular hydrogen bond between a water proton and the amino nitrogen). Interestingly, upon formation of this new network of hydrogen bonds a change in the conformation of the 2AE monomer also occurred. To accommodate insertion of the water molecule upon formation of the complex, an increase in the OCCN dihedral angle and in the ON heavy atom distance of 18° and 0.304 Å, respectively, of the 2AE monomer was observed. Such changes in monomer conformation upon complexation are rare. In general, molecules retain their conformation when forming complexes. On the basis of the increased strength of the intramolecular hydrogen bond in the six-membered and seven-membered rings found in 3AP and 4AB, one might expect more resistance to change in the conformation adopted by the amino alcohol monomer of these larger amino alcohols upon formation of the water complex than that observed in 2AE. The current combined experimental and theoretical study seeks to examine the interplay of intra- and intermolecular hydrogen bonds within the 3AP−H2O complex and discusses the role that increased intramolecular hydrogen-bond strength plays in the stabilization of conformation.
Figure 1. Resolved nuclear quadrupole hyperfine structure for the 423−322 rotational transition of 3AP−H2O. All lines consist of Doppler doublets because the microwave radiation in the cavity propagates parallel and antiparallel to the unidirectional molecular beam.
The center frequencies were fit to the Watson A-reduction Hamiltonian;16 the rotational and centrifugal distortion constants obtained for 3AP-H2O from the fits are given in Table 1. Table 1. Spectroscopic Constants of the 3-Aminopropanol− H2O Complex normal A/MHz B/MHz C/MHz ΔJ/kHz ΔJK/kHz δJ/kHz no. transitions std dev/kHz χaa/MHz χbb/MHz
2. EXPERIMENTAL SECTION Rotational spectra were measured using a pulsed molecular beam, Fabry−Perot cavity spectrometer described in detail elsewhere.12,13 Short survey scans (on the order of 100−500 MHz) were collected in order that the signal intensity from the 3AP monomer, H2O dimer, and potential 3AP-H2O transitions could be monitored. For each survey scan, 20 free induction decays were averaged and Fourier transformed at stepped frequency intervals of 0.5 MHz. Assigned transitions were later remeasured with additional averaging in order to fully resolve the hyperfine structure resulting from the quadrupolar nitrogen atom. The graphical user interface JB9514,15 was used to patch together short survey scans and individual measurements and to fit rotational, centrifugal distortion, and nuclear quadrupole coupling constants. 3-Aminopropanol, purchased from SigmaAldrich and used without further purification, was warmed to 70 °C in a heated reservoir nozzle oriented parallel to the cavity axis. In this configuration, rotational line widths are approximately 2 kHz. The 3AP vapor was expanded at a backing pressure of ∼1.5 atm using a 80%/20% mixture by volume of neon and helium carrier gas which contained ∼1% water vapor. The rotational temperature in the expansion under these conditions is ∼2 K.
3211.1441(3) 2379.8824(6) 1810.8366(5) 2.94(5) −1.94(5) 0.71(1) 15 1.7 −1.556(2) −1.308(2)
4. COMPUTATIONAL METHODS AND STRUCTURES Theoretical modeling of the 3AP−H 2 O complex was performed using the Gaussian 09 suite of programs.17 Geometry optimizations were carried out at the MP2/6-311+ +G(d,p) level including counterpoise correction (CP) using the counterpoise method of Boys and Bernardi18 with a tight convergence criterion. Starting structures of the 3AP monomer were based on two low energy conformations obtained from previous calculations5 (herein referred to as 3AP1 and 3AP2 and shown in Figure 2). The heavy atom backbone of 3AP is characterized by two independent torsional angles; τ 1 (NC3C2C1) and τ2 (C3C2C1O). Both low energy conformers of the 3AP monomer are stabilized by the same intramolecular hydrogen bonding arrangement, (OH····N), but differ in the orientation of the heavy atoms of the backbone with 3AP2 lying 8.09 kJ mol−1 above 3APl. The global minimum 3AP1 conformation resembles a chairlike configuration with carbon C(2) lying out of the plane determined by the other heavy atoms. The backbone dihedral angles τ1(NC3C2C1) and τ2(C3C2C1O) are 61.3° and −69.5°, respectively. The 1.994 Å intramolecular hydrogen bond formed by the interaction of the hydroxyl oxygen with the
3. RESULTS Fifteen a- and b-type rotational transitions were measured for the 3AP−H2O complex consisting of a total of forty-five nuclear quadrupole hyperfine components (representative hyperfine splitting is shown in Figure 1). The hyperfine transitions, available in Table S1 in the Supporting Information, were fit to the 15 rotational line centers and the nuclear quadrupole coupling constants: χaa = −1.556(2) MHz and χbb = −1.308(2) MHz (Δvrms = 2.4 kHz). B
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complexation with one water molecule results in low energy structures that contain a network of hydrogen bonds by water simultaneously acting as proton donor and acceptor. And most importantly, the most stable conformer found is based on the higher energy monomer conformer, 3AP2. Three minima for the 3AP−water complex based on the lowest energy 3AP1 monomer were found and are shown in Figure 3. Two of these, 3AP1−w1 and 3AP1−w2 contain a network of intermolecular hydrogen bonds while the third 3AP1−w3 contains a single intermolecular hydrogen bond. 3AP1−w1 was found to be the most stable of the three with a relative stabilization energy of 2.77 kJ mol−1 compared to the most stable dimer 3AP2−w1, discussed later. In 3AP1−w1, both the conformation and the intramolecular hydrogen bond of the monomer are preserved upon complexation (values of τ1 and τ2 in the complex are within 5% of those found in the 3AP1 monomer) while in 3AP1−w2 disruption of the intramolecular hydrogen bond is observed. In addition to the loss of the intramolecular hydrogen bond, a large change in the backbone configuration is observed with the backbone torsional angle τ2 increasing by 12.5% in order to incorporate the intermolecular hydrogen bonds formed with water. In 3AP−w3, an exocyclic hydrogen bonding configuration is observed with preservation of the backbone torsional angles and intramolecular hydrogen bond found in the 3AP1 monomer upon formation of a single intermolecular hydrogen bond with water donating to the 3AP1 oxygen atom. As with the case of complexes involving 3AP1, two classes of water complexes with 3AP2 were observed, those containing either a network (3AP2−w1 and 3AP2−w2) or single (3AP2− w3, 3AP2−w4, and 3AP2−w5) intermolecular hydrogen bonds. Interestingly, the most stable conformers of the 3AP−H2O complex found were based on the higher energy 3AP2 monomer, (labeled as 3AP2−w1 and 3AP2−w2 in Figure 4). The conformer 3AP2−w1 is stabilized over 3AP2−w2 by only 0.26 kJ/mol, and the inclusion of zero-point energy confirmed the stability of 3AP1−w1 over 3AP1−w2 (see Table 2). In both these complexes, the water molecule inserts itself between the amine and alcohol disrupting the intramolecular hydrogen bond of the monomer in favor of the formation of two new intermolecular hydrogen bonds. The primary difference between the two conformers is the orientation of the nonhydrogen-bonded water proton. The NHaOwHb torsional angles (where Ha and Hb represent the bonded and nonhydrogen-bonded water protons) were found to be 176.4° and −134.5° for 3AP2−w1 and 3AP2−w2, respectively. In addition to the loss of the intramolecular hydrogen bond, the backbone conformation of the 3AP2 monomer experiences large changes upon complexation with τ1(NC3C2C1) increasing by 39% and an increase in τ2(C3C2C1O) of 35%. In each of the conformers containing single intermolecular hydrogen bonds the intramolecular hydrogen bond and the backbone torsional angles of the monomer are preserved upon complexation. These conformers differ by the orientation of the exocyclic bound water molecule, which stays either above or below the 3AP2 molecule and whether water hydrogen bonds with the alcohol or amine groups in the amino alcohol. Interestingly, no minima based on the 3AP1 monomer with the exocyclic water below or above the ring (including either NH··· Ow or OwH···O) could be located with the applied method MP2/6-311++G(d,p) when the counterpoise correction was included in the geometry optimization.
Figure 2. Intramolecular hydrogen bond stabilized low energy conformers of the 3-aminopropanol monomer from MP2/6-311+ +G(d,p) calculations. Conformers are characterized by the two torsional angles τ1(NC3C2C1) and τ2(C3C2C1O) involving the heavy atom backbone atoms.
amino nitrogen results in an O−N heavy atom separation of 2.825 Å. The higher energy conformation, 3AP2, adopts an envelope-type structure with C(1) and C(3) lying on opposite sides of the plane formed by the remaining heavy atoms (τ1(NC3C2C1) = 43.5°, τ2(C3C2C1O) = 43.8°) giving a 2.315 Å intramolecular hydrogen bond and a O−N heavy atom separation of 2.983 Å. Starting structures for 3AP−H2O were based on these two low energy monomers. Formation of the 1:1 complexes of 3AP and water was accomplished by adding one water molecule at those sites in each monomer which are likely to participate in hydrogen bonding interactions followed by full geometry optimization including counterpoise correction. The stability of dimers can be described energetically by the pure interaction energy Eint of the two monomers in the dimer, by the binding energy EBE including the stabilization relative to the monomers, or better by the stabilization energy Estab, which is EBE corrected by the zero-point energy (ZPE). In contrast to the pure interaction energy, often used for molecular clusters, the binding energy also contains the deformation energy from the monomers, and therefore describes the energetic stability of the clusters more appropriately. In this work, we used the stabilization energy for comparison with the experimental data. All listed values of EBE, and consequentially all values of Estab, were CP-corrected. Optimized 3AP−water structures based on 3AP1 and 3AP2 monomers are shown in Figure 3 and Figure 4, respectively. The important structural parameters of both sets of complexes are listed in the Supporting Information (Tables S2 and S3) with energetic data given in Table 2. In both cases,
Figure 3. Molecular structures and relative stabilization energies of 3aminopropanol−H2O complexes based on 3AP1, optimized at the MP2/6-311++G(d,p) level; see also Table S2. C
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Figure 4. Molecular structures and relative stabilization energies of 3-aminopropanol−H2O complexes based on 3AP2, optimized at the MP2/6311++G(d,p) level; see also Table S3.
single intermolecular hydrogen bond. The individual rotational constants are collected in the Supporting Information Tables S2 and S3. The largest differences are found for conformations containing single intermolecular hydrogen bonds between water and the amino alcohol; these also correlate to the conformations having the highest energies. On this basis, these conformers can be removed from consideration. The remaining conformations all contain a network of intermolecular hydrogen bonds. 3AP1−w2, in addition to being energetically unfavorable, has calculated dipole moment components that are inconsistent with experiment. 3AP1−w2 is calculated to exhibit a strong c-type spectrum, however only a- and b-type transitions are observed. 3AP1−w2 is therefore also removed from consideration. Two unique conformers remain for consideration, 3AP1−w1 and 3AP2−w1. Recall that 3AP2−w1 and 3AP2−w2 are related by the orientation of the non-hydrogen-bonded water proton. The hydrogen bond network is similar to that found in the 2AE−H2O11 and alaninamide−H2O19 van der Waals complexes. The non-hydrogen-bonded water proton in these complexes undergoes a large amplitude rocking motion. Vibrational averaging along the water rocking coordinate results in a planar effective spectroscopic structure making it not possible to conclusively identify the location of the nonhydrogen-bonded water proton. The measured spectroscopic constants are in excellent agreement with those calculated for 3AP1−w1. The largest difference, occurring in B, is still within 1.5% of the measured value. The A and C rotational constants are in even better agreement with percent differences of 0.02 and 0.8, respectively. However, the calculated difference in rotational constants for 3AP2−w1 are an order of magnitude larger. In addition, the calculated dipole moments are consistent with 3AP1−w1. The experimental spectra are therefore assigned to conformer 3AP1−w1. Interestingly, while this complex is based on the lowest energy 3AP monomer conformation, it is 2.77 kJ/mol less stable than the global minimum 3AP2−w1, which is based on the higher energy monomer conformation. No experimental evidence of the higher energy 3AP2 monomer was found in the
Table 2. Energetic Parameters of 3-Aminopropanol−H2O Complexes Based on 3AP1 and 3AP2 Predicted by MP2/6311++G(d,p) Calculations kJ/mol 3AP1−w1 3AP1−w2 3AP1−w3 3AP2−w1 3AP2−w2 3AP2−w3 3AP2−w4 3AP2−w5
Eint
Eint,rel
EBE
Estab
Estab,rel
−33.61 −45.37 −26.82 −44.00 −44.38 −28.86 −24.86 −13.28
11.76 0.00 18.54 1.37 0.99 16.50 20.51 32.09
−31.81 −21.41 −26.14 −35.07 −34.62 −28.60 −24.69 −13.27
−22.76 −12.86 −18.81 −25.53 −25.32 −20.37 −17.40 −9.78
2.77 12.67 6.72 0.00 0.22 5.17 8.13 15.75
5. DISCUSSION The experimental spectroscopic constants of the 3AP−H2O complex are inconsistent with those predicted for several of the ab initio structures. Table 3 gives the percent difference between the experimental and calculated rotational constants (ΔA, ΔB, and ΔC) for all conformational minima of the complexes of water with each of the 3AP monomers. In the table they are grouped according to the type of hydrogen bond formed, either a network of intermolecular hydrogen bonds or a Table 3. Difference (%) between Experimental and Calculated Rotational Constants ΔA 3AP1−w1 3AP1−w2 3AP2−w1 3AP2−w2 3AP1−w3 3AP2−w3 3AP2−w4 3AP2−w5
ΔB
Intermolecular Hydrogen Bond Network 0.02 −1.5 0.3 7.3 −6.4 13.4 −4.7 11.9 Single Intermolecular Hydrogen Bond 43.0 −78.3 13.1 −18.8 50.6 −89.2 11.0 −37.1
ΔC −0.8 −6.2 −9.5 −9.4 −55.0 −1.8 −58.0 −18.1 D
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The Journal of Physical Chemistry A previous study of the 3-aminopropanol monomer5 nor in the current study. Although the complex involving the 3AP2 monomer is calculated to be more stable, the lack of that monomer in the expansion may preclude its formation. Unlike the case of 2-aminoethanol which sacrifices its intramolecular hydrogen bonding network to accommodate the formation of intermolecular hydrogen bonds with water, 3aminopropanol appears to maintain its intramolecular hydrogen bonds when forming 1:1 van der Waals complexes. This effect is presumably due to the increase in strength of the sixmembered ring of 3AP relative to the five-membered ring found in 2AE. This is consistent with the results of earlier liquid phase IR and Raman measurements9,10 which reported spectroscopic evidence of the presence of free monomers of 3AP in solution which had the preferred OH····N intramolecular hydrogen bonding configuration found in the gas phase. No such evidence was found in the case of 2AE. Within condensed phase amino alcohol aggregates, the conformation of the amino alcohol monomer is different than it is in isolated monomers. Within aggregates, the amino alcohol adopts a NH····O intramolecular hydrogen bonding arrangement. The breaking of the preferred OH····N intramolecular hydrogen bond in the isolated monomer to form NH····O intramolecular hydrogen bonds makes the hydroxyl group more acidic and the amino group more basic and serves to activate both toward the formation of the predominant OH····N intermolecular hydrogen bond found in the aggregate. The mean energy of stabilization of the OH····N intermolecular hydrogen bond is estimated to be 29 kJ/mol,20 which is nearly twice that of the intramolecular hydrogen bonds found in the free monomer, and therefore leads to their disruption. Current studies are underway for the 4-aminobutanol water complex. As the ring size of the amino alcohol increases from five and six in 2AE and 3AP respectively to seven in 4AB, a concomitant increase in the strength of the intramolecular hydrogen bond is expected. This may lead to a change in the intermolecular hydrogen arrangement from networked to singular intramolecular hydrogen bond.
the strength of the intramolecular hydrogen bond is proposed to be a function of the ring size.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b05745. Nuclear quadrupole hyperfine transition frequencies and the resulting unsplit line center after fitting to the quadrupole coupling constants (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: 843-953-5275. ORCID
Richard J. Lavrich: 0000-0003-2789-6436 Present Address #
A.S.K.: Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS R.J.L. acknowledges the financial support of the College of Charleston, Research Corporation (Single-Investigator Cottrell Science Award, and a grant to the College of Charleston from the Howard Hughes Medical Institute (52007537) through the Undergraduate Science Education Program
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REFERENCES
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CONCLUSIONS The rotational spectrum of the 3-aminopropanol−water complex has been measured. The amino and alcohol functional groups in amino alcohols may serve as either hydrogen-bond donor or acceptor. Two hydrogen bonding motifs of the 3aminopropanol monomer are possible; (i) alcohol proton to amino nitrogen (OH····N) and (ii) amino protons to alcohol oxygen (NH····O). Formation of the intramolecular hydrogen bond stabilizes the conformation and results in a ring-like structure consisting of the backbone carbon atoms and the amino and alcohol functional groups participating in the hydrogen bond. The strength of the intramolecular hydrogen bond has been investigated by introducing intermolecular hydrogen bonds with water. The experimental constants are consistent with an ab initio conformation calculated at the MP2/6-311++G(d,p) level that involves the lowest energy 3-aminopropanol monomer. The calculated global minimum ab initio complex however involves a higher energy monomer conformation of 3aminopropanol. In contrast to 2-aminoethanol, the intramolecular hydrogen bond is not sacrificed in the experimentally confirmed 3-aminopropanol but an additional intermolecular hydrogen-bond network occurs upon complexation. Therefore, E
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