Competing Intramolecular Hydrogen Bond Strengths and

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Competing Intramolecular Hydrogen Bond Strengths and Intermolecular Interactions in the 4-Aminobutanol-Water Complex Jenna A. Hohl, Michael W. Harris, Nina Strasser, Anne-Marie Kelterer, and Richard J Lavrich J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05888 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018

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The Journal of Physical Chemistry

Competing Intramolecular Hydrogen Bond Strengths and Intermolecular Interactions in the 4Aminobutanol-Water Complex Jenna A. Hohl1, Michael W. Harris1, Nina Strasser2, Anne-Marie Kelterer2, Richard J. Lavrich* 1,a

1. Department of Chemistry and Biochemistry, College of Charleston, 66 George St., Charleston, SC, USA, 29424 2. Institute of Physical and Theoretical Chemistry, NAWI Graz, Graz University of Technology, Stremayrgasse 9/Z2, Graz, Austria a) E-mail address: [email protected], tel. 843-953-5275

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ABSTRACT

We seek to determine the effect of competing intermolecular hydrogen bonds from water on the preferred conformation of 4-aminobutanol (4AB) monomers stabilized by intramolecular hydrogen bonds. Towards this end, the rotational spectrum of the 4-aminobutanol-H2O complex was recorded using Fourier-transform microwave spectroscopy and fit to the rotational, quadrupole coupling, and centrifugal distortion constants of the Watson S-reduction Hamiltonian. The experimental results are consistent with a 4AB-water complex that preserves the intramolecular hydrogen bond within the 4AB monomer and forms a single intermolecular bond with water acting as donor. The experimental monomer structure agrees well with the lowest energy conformation calculated at the MP2/6-311++G(d,p) level of theory. Upon complex formation and the introduction of competing intermolecular bonds from water, only small changes in the OH····N intramolecular hydrogen bond and backbone torsional angles of the 4aminobutanol monomer are observed. Similar small changes were observed for the shorter chain 3-aminopropanol amino alcohol monomer when complexed with water, in contrast to the 2aminoethanol-H2O complex. In the latter, a large change in the backbone torsional angle and a breaking of the intramolecular hydrogen bond were observed. Thus, extending the methylene chain results in an increase in the strength of the intramolecular hydrogen bond in unbranched amino alcohols.

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1. INTRODUCTION

Unbranched amino alcohols, HO(CH2)nNH2, contain functional groups capable of forming hydrogen bonds separated by a backbone methylene (CH2) chain. As a result, monomers of amino alcohols adopt conformations that are stabilized by intramolecular hydrogen bonding. Although two unique types of intramolecular hydrogen bonding are possible; alcoholto-amine (OH ····N) and amine-to-alcohol (NH····O), gas phase studies of isolated monomers of 2-aminoethanol (2AE),1-4 3-aminopropanol (3AP),5-8 4-aminobutanol (4AB),9 and 5aminopentanol (5AP)10 show evidence of conformers containing only alcohol-to-amine intramolecular hydrogen bonds. The strength of the intramolecular hydrogen bond is proposed to be a function of the length of the backbone methylene chain separating the amino and alcohol groups. The origin of the increase in hydrogen bond strength as a function of increasing length of the methylene chain is thought to be due to an increase in conformational flexibility with longer chain amino alcohols better able to position the amino and alcohol groups so as to maximize the stabilizing hydrogen bonding interaction. This increase in strength of the intramolecular hydrogen bond with backbone chain length has been inferred from IR studies of amino alcohols. For isolated monomers in the gas phase, a larger red shift of the OH stretch was observed in spectra of 3AP8 relative to 2AE3 indicating an increase in the strength of the intramolecular hydrogen bond. Further evidence comes from condensed phase studies. Within the condensed phase aggregate,11,12 the amino alcohol monomer adopts an NH····O intramolecular hydrogen bond. This change in preferred hydrogen bonding motif results in a more acidic alcohol group and more basic amino group which serves to activate the monomers toward the formation of the predominant OH ····N intermolecular hydrogen


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bonds found between monomers in the aggregate. The formation of aggregates therefore requires a breaking of the preferred OH····N intramolecular hydrogen bond found in the isolated monomer. In condensed phase IR and Raman spectra of 3AP,11 spectral features arising from the aggregate as well as from free monomers of 3AP with intact OH····N intramolecular hydrogen bonds were observed. No such monomer signal was found in studies of 2AE12 indicating a weaker intramolecular hydrogen bond that is completely sacrificed in the process of aggregate formation. The lack of free monomers of 2AE in condensed phase spectra therefore lends support to the trend of increased intramolecular hydrogen bond strength with increasing backbone chain length. Ab initio studies13 lend additional support to the above conclusion. In comparison with 2AE, a decrease in the OH····N intramolecular hydrogen bond distance of 11.2% and a corresponding increase in the O-H bond length of 0.2% was calculated for the 3AP monomer indicating a stronger intramolecular hydrogen bond. Larger changes were observed when comparing 2AE with 4AB, with the intramolecular hydrogen bond distance decreasing by 18.6% and the hydroxyl bond length increasing by 0.8%. In the current study, Fourier-transform microwave spectroscopy was used to probe intramolecular hydrogen bond strength by comparing the conformation adopted by the free amino alcohol monomer 4AB to that within the 1:1 complex with water. The presence of competing intermolecular hydrogen bonds from water has the potential to disrupt the intramolecular hydrogen bond and/or cause conformational changes in the amino alcohol monomer. Previous microwave studies of 2AE-H2O14 and 3AP-H2O15 suggest that as the size of the amino alcohol monomer increases, so does its ability to preserve its intramolecular hydrogen bond and resist changes in conformation when complexed with water. In the case of 2AE-H2O, the intramolecular hydrogen bond in the monomer is completely sacrificed in favor of two new ACS Paragon Plus Environment

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intermolecular hydrogen bonds upon complexation with water (Figure 1a). Along with the breaking of the intramolecular hydrogen bond comes a large change in the conformation of the 2AE monomer where the τ(NC2C1O) torsional angle of the heavy atom backbone increases by 18º upon formation of the 2AE-H2O complex.

Figure 1. Conformation of (a) 2-aminoethanol (2AE) and (b) 3-aminopropanol (3AP) before and after complexation with H2O. Large changes in the 2AE amino alcohol monomer conformation and disruption of its intramolecular hydrogen bond are observed upon introduction of competing intermolecular hydrogen bonds from water. For the longer chain 3AP monomer, smaller changes in conformation and a preservation of the intramolecular hydrogen bond occur upon complexation.


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In the case of 3AP, with a presumably stronger intramolecular bond, the formation of the 1:1 complex, with the addition of new OwHw····O and NH····Ow intermolecular hydrogen bonds from water, yields a structure (see Figure 1b.) in which the 3AP monomer retains its intramolecular hydrogen bond and maintains its conformation with only small changes in the backbone torsional angles used to characterize it. Upon introduction of the competing intermolecular hydrogen bonds during formation of the 3AP-H2O complex, the τ1(NC3C2C1) and τ2(C3C2C1O) torsional angles change by only 5º and 3º, respectively. The current study examines the next larger amino alcohol in the series, 4-aminobutanol. As with the other isolated amino alcohols, the 4AB9 monomer is stabilized by an alcohol-toamine intramolecular hydrogen bond and characterized by a set of backbone torsional angles analogous to those used to describe the smaller chain 2AE and 3AP monomers. Due to the increase in the length of the methylene chain separating the amino and alcohol groups involved in the intramolecular hydrogen bond (from two to three to four in the series 2AE, 3AP, 4AB) and the presumed increase in the strength of the resulting intramolecular hydrogen bond, one would expect the 4AB monomer to be even more resistant to changes in conformation with the introduction of intermolecular hydrogen bonds when complexed with water. The present study seeks to confirm this expectation.


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2. EXPERIMENTAL SECTION Rotational spectra were measured using a pulsed molecular beam, Fabry-Perot cavity spectrometer described in detail elsewhere.16,17 Short survey scans (on the order of 100 -500 MHz) were collected in order that the signal intensity from the 4AB monomer, H2O dimer, and potential 4AB-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 hyperfine structure resulting from the quadrupolar nitrogen nucleus. The graphical user interface JB9518,19 was used to patch together short survey scans and individual measurements and to assign rotational quantum numbers to the energy levels involved in the rotational transitions. 4-Aminobutanol, purchased from Sigma-Aldrich and used without further purification, was warmed to 100 ºC in a heated reservoir nozzle oriented parallel to the cavity axis. In this configuration, rotational linewidths are approximately 15 kHz (full width at half-maximum), with line centers accurate to ±2 kHz. The 4AB 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. 3. RESULTS Fifteen a- and b-type rotational transitions, comprised of a total of forty-seven nuclear quadrupole hyperfine components, were measured for the 4AB-H2O complex. The hyperfine transitions are available as Supporting Information (Table S1). The results of a global fit of rotational, centrifugal distortion, and nuclear quadrupole hyperfine coupling constants performed with Pickett’s SPFIT20,21 program with a Watson S-reduction Hamiltonian22 are given in Table 1.


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Table 1. Spectroscopic constants of the 4-aminobutanol-H2O complex

Normal A / MHz

2191.484(2)a

B / MHz

1869.4204(5)

C / MHz

1445.0788(3)

DJ / kHz

3.351(8)

DJK / kHz

2.46(1)

DK / kHz

-6.7(2)

d1 / kHz

-1.022(5)

d2 / kHz

-0.447(6)

χaa / MHz

-0.19(1)

χbb / MHz

-1.95(1)

σb / kHz

1.8

Nc

47

aErrors

in parentheses are in units of the last digit. bRMS error of the fit. of lines in the fit

cNumber


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4. COMPUTATIONAL METHODS Starting structures of the 4AB monomer were based on three low energy conformations obtained from previous calculations9 (herein referred to as 4AB1, 4AB2, and 4AB3 and shown in Figure 2) and are characterized by a set of backbone torsional angles τ. For each monomer of 4AB, starting structures of the 4AB-H2O complex having (i) single OH····Ow, NH····Ow, OwHw····O, and OwHw····N as well as (ii) networks of OH····Ow/OwHw····N and NH····Ow/OwHw····O intermolecular hydrogen bonds were considered. All geometries were optimized by Møller-Plesset perturbation theory of second order (MP2) using the triple-zeta 6311++G(d,p) basis set.23,24 Counterpoise correction (CP) of the basis set superposition error (BSSE) was included in the geometry optimizations.25 Frequency calculations were performed to confirm the optimized geometries as true minima. The calculated relative (E), interaction (Eint), and dissociation (De) energies were corrected for zero-point energy (ZPE) (Eo and Do) and for BSSE (E + BSSE). The program GAUSSIAN0926 was used throughout this study.


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Figure 2. Intramolecular hydrogen bond stabilized low energy conformers of the 4-aminobutanol monomer from MP2/6-311++G(d,p) calculations. Conformers are characterized by three torsional angles τ1(NC4C3C2), τ2(C4C3C2C1), and τ3(C3C2C1O) involving the heavy atom backbone. 5. COMPUTED STRUCTURES In this section, we present the stable 4AB-water dimers and discuss their backbone geometries, hydrogen bonding and energetics. The calculated minima based on the 4AB1, 4AB2, and 4AB3 monomers are shown in Figure 3 with important spectroscopic and energetic parameters listed in Table 2 for the two experimentally relevant complexes and in the Supporting Information Tables S2 and S3 for all conformations.


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Figure 3. Molecular structures of 4-aminobutanol-H2O complexes optimized at the MP2/6311++G(d,p) level; see also Tables S2 and S3. Table 2. Calculated MP2/6-311++G(d,p) energetica and spectroscopic data of the two spectroscopically relevant 4-aminobutanol-H2O complexes 4AB1-w1 and 4AB3-w3. 4AB1-w1

4AB3-w3

E / kJ/mol

0.00

8.03

E0 / kJ/mol

0.00

9.63

E0 / cm-1

0.00

805.1

(E+BSSE) / kJ/mol

0.00

8.56

Eint / kJ/mol

11.99

0.00


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De / kJ/mol

11.07

0.00

D0 / kJ/mol

10.22

0.00

A / MHz

2201.704

2175.188

B / MHz

1810.534

2059.326

C / MHz

1436.577

1338.107

|a| / D

2.28

3.52

|b| / D

0.68

0.06

|c| / D

0.53

0.67

χaa / MHz

-0.249

-1.814

χbb / MHz

-1.929

0.291

and dissociation energies are BSSE corrected; E…relative energy, E0…relative energy with zero point energy correction (ZPE), E+BSSE)… relative BSSE-corrected energy, Eint…relative interaction energy, De…relative dissociation energy without ZPE, D0…relative dissociation energy with ZPE.

aInteraction

For the complex containing the global minimum 4AB1 monomer, two unique conformations were found. In both, the OH····N intramolecular hydrogen bond of the monomer is preserved upon complexation with slightly shortened bond lengths (1.782 Å and 1.755 Å in 4AB1-w1 and 4AB1-w2, respectively, relative to 1.846 Å in the monomer). The two minima differ in the type of intermolecular hydrogen bonding motif they contain; a single OwHwO intermolecular bond (1.922 Å) in 4AB1-w1 versus a network of OwHwO (1.994 Å) and NHOw (2.482 Å) intermolecular hydrogen bonds in 4AB1-w2.

Accompanying this difference in


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intermolecular hydrogen bonding motif is a change in the conformation adopted by the 4AB1 monomer. In 4AB1-w1, the conformation of the 4AB1 monomer is essentially unchanged upon complexation (the largest change occurs in the τ3(C3C2C1O) torsional angle which decreases by 4%). In 4AB1-w2 larger changes were observed with the τ1(NC4C3C2) and τ2(C4C3C2C1) torsional angles changing by 12.8% and 22 %, respectively, to accommodate the water. Similar interaction energies for 4AB1-w1 and 4AB1-w2 were calculated (-34.16 kJ/mol and -34.57 kJ/mol respectively) but with an additional 4.29 kJ/mol required for the deformation of the monomers in 4AB1-w2. For the 4AB2 monomer, all three calculated minima of the complex contain single OwHw····O intermolecular bonds. In each of the complexes, the 4AB2 monomer is essentially unchanged with all torsional angles within a few degrees of those found in the monomer. The complexes differ primarily in the location of the water which is oriented either to the side (4AB2w1), below (4AB2-w2), or above (4AB2-w3) the 4AB2 monomer. As found in the case of complexes involving 4AB1, the intramolecular hydrogen bond of the 4AB2 monomer (1.981 Å) is preserved and somewhat shortened upon complexation (1.910 Å, 1.891 Å, 1.839 Å for 4AB2w1, 4AB2-w2, and 4AB2-w3 respectively). All three minima have interaction energies of approximately -30 kJ/mol with the slightly shorter (and stronger) OwHw····O intermolecular hydrogen bond in 4AB2-w2 consistent with the energetics. Optimization of conformers, beginning with a network of intermolecular hydrogen bonds between 4AB2 and water. converged either to 4AB2-w2, where the network was replaced with a single OwHw····O intermolecular hydrogen bond, or to 4AB1-w2, in which the network of intermolecular hydrogen bonds was maintained but the amino alcohol conformation changed from 4AB2 to 4AB1.


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For the highest energy 4AB3 monomer, three 4AB-water dimer minima were found. 4AB3-w1 and 4AB3-w2 contain single OwHw····O intermolecular hydrogen bonds with water above and below the 4AB monomer, respectively. Both conformations have interaction energies similar to that found for the analogous hydrogen bonding type found in conformations of 4AB1water and 4AB2-water dimers (around -30 kJ/mol). In each, a preservation of the OH····N intramolecular hydrogen bond and small changes in the torsional angles (on the order of 2%) of the 4AB3 monomer is observed. The slightly greater interaction energy of 4AB3-w2 relative to 4AB3-w1 (-33.62 kJ/mol vs. -28.95 kJ/mol) is the result of shorter and stronger inter- and intramolecular H-bonds (see Table S2 in the SI). 4AB3-w3 contains a network of OH····Ow (1.998 Å) and OwHw····N (1.939 Å) intermolecular hydrogen bonds. As with the networked configuration based on 4AB1, a similar large change in the torsional angles of the 4AB3 monomer was observed upon complexation (with changes between 20% to 40%). Unlike 4AB1-w2 however, the intramolecular hydrogen bond in 4AB3 is completely sacrificed in favor of the two new intermolecular hydrogen bonds with water. Interestingly, despite containing the highest energy amino alcohol monomer, 4AB3-w3 is calculated to be the most stable of all the 4AB-water complexes having the greatest interaction and highest dissociation energy compared to all other 4AB-water conformations. This strong stabilization is the result of stronger hydrogen bonds produced when water acts as both acceptor and donor in the complex. Several key points can be summarized from the computational results. First, although 4AB1-w1 has the lowest relative energy of all complexes, the interaction energy (describing the stabilization in the dimer complex) and the dissociation energy (including relaxation from the monomer geometries) point to a stronger interaction of the monomers upon complex formation in


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4AB-w3. Second, the 4AB monomer in complexes containing one single intermolecular hydrogen bond exhibits small changes in conformation upon complexation. Much larger changes in the 4AB monomer conformation are observed for complexes containing a network of intermolecular hydrogen bonds. Finally, a 3:1 preference for conformations containing a single OwHw····O intermolecular hydrogen bond over the networked arrangement is observed. This represents a shift in the type of intermolecular hydrogen bonding arrangement preferred by the 4AB monomer upon complexation relative to 3AP. For the 3AP-H2O complex,15 the calculated minima were evenly distributed amongst the two intermolecular hydrogen bonding motifs. This increase in the relative number of conformations of 4AB-H2O that contain single intermolecular hydrogen bonds may suggest a stronger intramolecular hydrogen bond in the 4AB monomer relative to 3AP. 6. DISCUSSION Table 3 gives the percent difference between the experimental and calculated rotational constants (∆A, ∆B, and ∆C) for all conformational minima of the 4AB-H2O complex with each of the 4AB monomers considered in the study (individual rotational constants are collected in the Supporting Information Table S3). Table 3. Difference (%) between experimental and calculated rotational constants

∆A

∆B

∆C

4AB1-w1

-0.5

3.1

0.6

4AB1-w2

-12.6

7.1

13.6


15


4AB2-w1

-68.3

42.6

37.9

4AB2-w2

-15.0

20.3

2.5

4AB2-w3

-31.3

11.8

16.9

4AB3-w1

-62.1

38.2

30.8

4AB3-w2

-18.7

9.5

12.5

4AB3-w3

0.7

-10.2

7.4

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Based on the comparison of these differences, we conclude that only two calculated conformations, 4AB1-w1 and 4AB3-w3, remain as candidates to be assigned to the experimentally detected structure. Furthermore, the experimental rotational constants are in better agreement with those calculated for 4AB1-w1 with the A and C rotational constants calculated to within ~0.5% of the experimental value. The largest difference, occurring in B, is still within 3% of the experimental value. Larger differences are observed for 4AB3-w3 where both the calculated rotational constants B and C differ by approximately 10% from those determined from the spectroscopic fits. Additionally, much larger differences between calculated (Table 2) and experimental (Table 1) nuclear quadrupole hyperfine coupling constants (χaa and χbb) are observed for 4AB3-w3 with those calculated for 4AB1-w1 in excellent agreement with experiment. Finally, the observation of btype transitions is inconsistent with the calculated dipole moments of 4AB3-w3. The experimental spectra are therefore assigned to conformer 4AB1-w1.


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Despite the higher energy 4AB3 monomer forming the most stable complex with water, we conclude that the global minimum conformation of the monomer (4AB1) is preserved during complexation.

Similar behavior was observed for N-methylformamide-H2O28 and 3AP-H2O15

where the most stable complex was calculated to contain a higher energy monomer. Using the relative energies of the three 4AB monomer conformations considered in this study, along with the temperature of the heated reservoir nozzle (~373 K), the Boltzmann distribution was used to provide an estimate of the pre-expansion populations of each 4AB monomer. It is estimated that prior to expansion, the populations of the 4AB2 and 4AB3 monomers was 16% and 8% respectively. It has been shown that conformational relaxation to the most stable conformer will occur during the supersonic expansion if the interconversion barrier is smaller than 2kT where T is the pre-expansion temperature.27

In the current study, this barrier limit is estimated to be 518

cm-1. The relative energies9 separating 4AB2 and 4AB3 from the global minimum 4AB1 are calculated to be 407 cm-1 and 592 cm-1 respectively.

It seems likely that even if some of the

4AB3 monomer calculated to be present in the pre-expansion mix were to survive the cooling process, its population would be low. No spectral features belonging to conformers other than the 4AB1 monomer were detected in the current study or in the previous study of the monomer.9 Comparing our result for 4AB with the previous works on 3AB and 2AE,13-15 a decrease in the changes in the backbone torsional angles characterizing the amino alcohol monomer in response to the introduction of competing intermolecular hydrogen bonds from water as a function of backbone chain length has been observed. For 2AE, the backbone torsional angle τ(NC2C1O) increased by 32% upon formation of the 2AE-H2O complex.14 In addition to this large change in the 2AE monomer backbone, the stabilizing OH····N intramolecular hydrogen bond is completely sacrificed in favor of two new intermolecular hydrogen bonds with water in a networked arrangement (see Figure 1a.) The larger 3AP monomer exhibits much smaller ACS Paragon Plus Environment

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changes in its backbone upon complexation to form 3AP-H2O, with τ1(NC3C2C1) and τ2(C3C2C1O) changing by 7% and 4% respectively, and a preservation of the OH····N intramolecular hydrogen bond of the monomer during the formation of network of OwHw····O and NH····Ow intermolecular bonds with water.15 In the 4AB-H2O complex, even smaller changes in the backbone torsional angles of the monomer were observed upon complexation; τ1(NC4C3C2) and τ2(C4C3C2C1) changed by 2% and 1% respectively with a change of 4% in τ3(C3C2C1O). The larger change in the τ3 torsional angle is presumably due to the single intermolecular hydrogen bond interaction of water with the alcohol oxygen. CONCLUSIONS A resistance to change the backbone conformation of amino alcohol monomers in response to the addition of competing intermolecular hydrogen bonds from water has been observed as a function of increasing backbone chain length. Such resistance is attributed to the increase in the intramolecular hydrogen bond strength with respect to increasing backbone chain length in the amino alcohol monomer. Such behavior also is suggested from earlier work3,8,11-13 on the amino alcohol monomers, e.g. by a red-shift of the OH stretching band indicating a strengthening of the intramolecular hydrogen bond with ring size. With this work and previous publications on amino alcohol-water complexes,7,9,14,15 it is also confirmed that the size of the amino alcohol backbone supports the same effect in amino alcohol-water dimers. As the number of methylene CH2 groups separating the intramolecular bound amino and alcohol groups increases, so does the flexibility to orient to optimize the hydrogen bond interaction. ASSOCIATED CONTENT


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Supporting Information. The Supporting Information contains nuclear quadrupole hyperfine transition frequencies as well as spectroscopic constants and relevant structural parameters for each of the ab initio conformations of the 4AB-H2O complex. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(Richard J. Lavrich). E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT AMK thanks Sigrid Haan and Theresa Köpping for generating the starting structures of 4AB-water dimers during their TU Graz internship in the framework of “Teens treffen Technik” for female pupils. RJL would like to acknowledge Stewart Novick, Helen Leung, and Frank Lovas for helpful discussions concerning Pickett’s SPFIT program.


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(13) Kelterer, A. M.; Ramek, M. Intramolecular Hydrogen Bonding in 2-Aminoethanol, 3Aminopropanol, and 4-Aminobutanol. J. Mol. Struct. Theochem. 1991, 232, 189-201. (14) Tubergen, M. J.; Torok, C. R.; Lavrich, R. J. Effect of Solvent on Molecular Conformation: Microwave Spectra and Structures of 2-Aminoethanol van der Waals Complexes. J. Chem. Phys. 2003, 119, 8397-8403. (15) Khalil, A. S.; Kelterer, A. M.; Lavrich, R. J. Interplay of Intermolecular and Intramolecular Hydrogen Bonds on Complex Formation: The 3-Aminopropanol-Water van der Waals Complex. J. Phys. Chem. A 2017, 121, 6646-6651. (16) Suenram, R. D.; Lovas, F. J.; Pereyra, W.; Fraser, G. T.; Hight Walker, A. R. Rotational Spectra, Structure, and Electric Dipole Moments of Methyl and Ethyl tert-Butyl Ether (MTBE and ETBE). J. Mol. Spectrosc. 1997, 181, 67-77. (17) Suenram, R. D.; Grabow, J. U.; Zuban, A.; Leonov, I. A Portable, Pulsed-Molecular-Beam, Fourier-Transform Microwave Spectrometer Designed for Chemical Analysis. Rev. Sci. Instrum. 1999, 70, 2127-2135 (18) Majewski, W. A.; Pfanstiel, J. F.; Plusquellic, D. F.; Pratt, D. W. In Laser Techniques in Chemistry, Myers, A. B., Rizzo, T. R., Eds; Wiley: New York, 1995; Vol. XXIII, pp. 101–148. (19) Plusquellic, D. F.; Suenram, R. D.; Mate, B.; Jensen, J. O.; Samuels, A. C. The Conformational Structures and Dipole Moments of Ethyl Sulfide in the Gas Phase. J. Chem. Phys. 2001, 115, 3057-3067. (20) Pickett, H.M. The Fitting and Prediction of Vibration-Rotation Spectra with Spin Interactions. J. Mol. Spectrosc. 1991, 148, 371-377. (21) Novick, S. E. A Beginner’s Guide to Pickett’s SPCAT/SPFIT. J. Mol. Spectrosc. 2016, 320, 1-7. (22) Watson, J. K. G. Determination of Centrifugal Distortion Coefficients of Asymmetric-Top Molecules. J. Chem Phys. 1967, 46, 1935-1949. (23) Raghavachari, K.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. 20. Basis Set for Correlated Wave-Functions. J. Chem. Phys. 1980, 72, 650-654. ACS Paragon Plus Environment

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Competing Intramolecular Hydrogen Bond Strengths and

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