Structure and Spectroscopy of Furan:H2O Complexes - The Journal of

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry 2

Structure and Spectroscopy of Furan:HO Complexes Schuyler P. Lockwood, Tyler G. Fuller, and Josh J. Newby J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06308 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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The Journal of Physical Chemistry

Structure and Spectroscopy of Furan:H2O Complexes Schuyler P. Lockwood, Tyler G. Fuller, and Josh J. Newby∗ Department of Chemistry, Hobart and William Smith Colleges, Geneva, NY 14456 E-mail: [email protected] Phone: 315-781-3757 Abstract An analysis of the 1:1 complex of furan and water is presented. In this study, computation and matrix isolation FTIR were used to determine stable complexes of furan : water. Density functional theory and Møller-Plesset second-order, perturbation theory calculations found four, unique geometries for the complex. Two complexes were characterized by C−H···O interactions, one by O−H···O, and the fourth by O−H···π. Optimizations completed using B3LYP, B3LYP-GD3BJ, M05-2X, and MP2 showed the most stable species to be bound by O−H···O interactions. Matrix isolation experiments of mixtures of furan and water held in nitrogen at 15 K showed evidence of stable complexes when probed by FTIR. These signatures grew in intensity when matrices were annealed at 30 K. These vibrational features were predominately associated with perturbation of the water monomer. Additionally, the spectra of complexes containing water isotopologues were recorded. Analysis of spectral features pointed to the presence of a single geometry formed in the matrix, which is best described as a 1:1 complex stabilized by a O−H···O interaction.

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Introduction Weakly bound complexes have been the subject of study for many years and have been studied by a wide variety of techniques. 1–5 The degree of study in this area is related to the prevalence of this behavior in nature (e.g. biochemical process, 6,7 the atmosphere, 8 surface science, 9 etc.). By studying and understanding simple molecules, we can build up an understanding of larger, more complicated systems. A common system of study is that of an aromatic molecule interacting with other small molecules. Part of the intrigue of this type of system is the wide variety of interactions that can occur, including basic hydrogen bonding, π interactions, and van der Waals attractions. One of the most studied complexes involving aromatics is that of benzene and water. This complex has been studied using many spectroscopic and computational methods with sizes ranging from a simple 1:1 complex 10–13 to the interaction of benzene with many water molecules. 14,15 The simplest complex (1:1) is described as a OH···π interaction. From this prototypical system, there are a multitude of studies that radiate outward to different perturbations of this system including substituted benzene rings, such as styrene (C6 H5 C2 H3 ), 16 phenol (C6 H5 OH), 17,18 aniline (C6 H5 NH2 ), 19 and fluorobenzenes (C6 H6-x Fx where x = 1 - 6). 20,21 The interactions of benzene have also been studied with many molecules other than water, including methanol (CH3 OH), 22 ammonia (NH3 ), 23 carbon monoixde (CO), 24 and acetylene (C2 H2 ). 25 In these complexes, a wide variety of interaction motifs have been observed and show the subtle interplay between the intermolecular forces present in each species. Heterocyclic aromatics (e.g. C4 H4 O, C4 H5 N, C4 H4 S) are also present in the literature, but to a lesser degree. Microwave spectroscopy has been used to study furan with SO2 , 26 C2 H4 , 27 and CO. 28 The furan : water complex is briefly mentioned in the publication devoted to the furan : carbon monoxide complex, 28 but there appears to be no dedicated publication on the water complex. The abstract of a related conference paper states that the water complex was identified in a microwave study and was determined to likely have a near planar geometry 2


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with large amplitude motions. 29 Unfortunately, rotational constants were not reported in the abstract, so little else can be inferred about the geometry of this complex. Matrix isolation FTIR spectroscopy has been used to determine the structure of furan complexes with methanol, 30 formic acid, 31 and acetylene, 32 but again, the complex involving water is noticeably missing. A single computational study on furan : water interactions showed two potential, complex geometries characterized by O−H···O or O−H···π interactions. 33 The aim of the current study is to fill in this gap in the literature on furan complexes. Here, we present an investigation of the weakly bound complexes of furan (C4 H4 O) with water as interrogated by matrix isolation FTIR and computational methods. Potential geometries of the 1:1 complex of furan : water (Fu:H2 O) will be presented and evidence will be given to support an experimental structure for this complex.

Methods Matrix Isolation The matrix isolation system (recently constructed at Hobart and William Smith Colleges) is based on standard designs and a description will be given here. Cryogenic temperatures of the sample are maintained using a CTI-Cryogenics, Model 22 Cryodyne (Helix Technology Corp.), backed by a 8200 series compressor. A window mount is attached to the cold finger and holds a 25 mm CsI window. The edges of the window were wrapped in indium foil (Sigma-Aldrich) to insure good thermal contact with the holder. Temperature of the cold finger was measured with a silicon diode monitored by a temperature controller (Lakeside, 335). Under optimum conditions, the cold finger can achieve temperatures as low as 8 K. The sample window and cold finger are contained in a vacuum chamber (Janis, CCS350R). This chamber has four, o-ring sealed optical ports and two o-ring sealed sample injection ports on a rotatable shroud. For FTIR spectroscopy experiments, two opposing optical ports are fitted with KBr windows. The two injection ports are at 45◦ angles to the 3



CsI cold window when the shroud is in deposition mode. High vacuum (< 10−6 mbar) is generated by an oil diffusion pump (Varian, HS-2). A liquid nitrogen trap is used to keep oil from back-streaming into the sample chamber. Pressure in the vacuum system is monitored by an ion gauge (Granville Phillips). The diffusion pump affords a base pressure for the chamber of 10−6 mbar. This pressure drops to 10−7 mbar when the cryogenic system is operational. The vacuum system is also connected to a stainless-steel sample preparation manifold. Samples are kept in glass, drying tubes that connect to the manifold using Ultra-Torr fittings. Pressure in the sample line is monitored using standard, digital pressure gauges (Ashcroft). Samples are prepared and stored in 3 L, stainless-steel, storage cylinders. Samples are directed by stainless-steel tubing to the sample injection ports. Fine-metering valves are used to control the deposition rate of samples. Furan (> 99 %, Sigma Aldrich) was dried over anhydrous magnesium sulfate prior to introduction to the vacuum system. Water (18 MΩ ) was obtained from an in-house purifier. Isotopic studies utilized D2 O (99.9 % D, Sigma Aldrich). All samples were degassed using standard freeze-pump-thaw cycling before use. Mixtures of furan, water, and matrix gases (nitrogen or argon, 99.9999 %, Linde) were prepared using standard manometric techniques. Samples were deposited at 15 K at a rate of 5 - 10 mm/hr for 4 - 8 hrs. Matrices were annealed at 30 K (for N2 ) or 35 K (for Ar) for 15 minutes. Annealing was used to soften the matrix and allow for diffusion in order to facilitate complexation. Sample spectra were recorded on a nitrogen purged, Nicolet iS50 (ThermoScientific) FTIR using 128 scans at 0.5 cm−1 resolution in the 4000 - 600 cm−1 region.

Computational Details Ground state geometries, energies, and harmonic frequencies were calculated using the Gaussian09 suite of programs. 34 Multiple levels of theory were employed, including density functional theory (DFT) and Møller-Plesset second-order, perturbation theory (MP2) 35 calcula4


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tions. DFT calculations employed the B3LYP, 36,37 M05-2X, 38 and ω-B97XD 39 functionals. As B3LYP does not account for dispersion forces, empirical corrections were made using GD3BJ. 40 Calculations were performed with the 6-311++G(d,p) Pople type basis set. 41 Tight convergence criteria and an ultra-fine grid were employed in all density functional calculations. Energies for cluster systems were corrected for zero point energy (ZPE) as well as basis set superposition error (BSSE), using the procedure of Boys and Bernardi. 42 A wide variety of complex geometries were sampled to best ensure that our study found the lowest energy configurations. Harmonic vibrational frequencies were scaled by 0.960 for OH stretches and 0.988 for all other vibrations. These scaling factors were chosen as they well represented the experimental vibrational frequencies of the monomers and are well within the norms presented in the literature. 43,44 Atoms in molecules (AIM) analysis was used to find bond critical points (BCPs), ring critical points (RCPs), and associated electron density values. This analysis was performed using AIMAll. 45

Results and Discussion Calculation Fu:H2 O Structures Four complex geometries were found across all levels of theory employed (Figure 1). Complexes I and II can be characterized as the oxygen of water binding to a C−H in a nonstandard hydrogen bond interaction. Complex III is a conventional hydrogen bond with an OH of water interacting with the oxygen of furan. Complex IV is characterized as an OH···π interaction on one side of the furan ring, thus leading to an equivalent, mirror image structure. This mirror image will not be discussed here, as it is indistinguishable in our spectroscopic studies. Complex I - III all have a CS geometry whereas complex IV is C1 . The ω-B97XD

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calculation did not yield the same structure for complex IV as all other levels of theory. In this calculation, the minimum energy structure was found to have the water monomer in the middle of the ring (CS geometry). This structure was found to be the transition state between the two mirror images using all other levels of theory. The calculated structures are otherwise consistent across B3LYP, B3LYP-GD3BJ, M05-2X, and MP2 levels of theory. For simplicity, discussion of parameters will be those from B3LYP, unless otherwise noted. Selected geometric parameters are given in Table 1. It should be noted that the study of Kaur 33 did not include complexes similar to structures I and II. It is unclear if the study did not find these geometries due to the higher energy of the structures while using a smaller basis set (6-31+G(d)) or if they were simply not included due to the nonstandard nature of this interaction. Complex III is in good agreement with Kaur’s 1-H2 O-A structure. 33 Additionally, complex IV is similar to Kaur’s 1-H2 O-B structure 33 in that both show an OH interaction with the π cloud of the furan ring. When complex IV is optimized at the level and basis set (MP2/6-31+G(d)) as used in Kaur’s study, the 1-H2 O-B geometry is generated. 33 I

II

Cα2 Cβ2

Cα1 Cβ1

+ III

IV

Figure 1: Calculated structures of the Fu:H2 O complex. All structures were optimized using B3LYP/6-311++G(d,p). All other levels of theory produced geometries very similar to those shown here. For labeling purposes, α carbons indicate the carbons closest to the oxygen, whereas β indicates the farther carbons. The 1 and 2 designations indicate if the carbon is on the same side (1) as the complexing water or on the opposing side (2).

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Table 1: Selected geometric parameters of Fu:H2 O complexes calculated at the B3LYP and MP2 levels of theory.a,b I CH···O Cα1 −H Cβ1 −H Cβ2 −H Cα2 −H

B3LYP 2.297 1.079 1.078 1.078 1.077

MP2 2.291 1.080 1.080 1.080 1.079

II CH···O Cα1 −H Cβ1 −H Cβ2 −H Cα2 −H

B3LYP 2.361 1.077 1.080 1.079 1.077

MP2 2.335 1.079 1.081 1.081 1.079

Furan

B3LYP

MP2

Cα1 −H Cβ1 −H Cβ2 −H Cα2 −H

1.077 1.078 1.078 1.077

1.079 1.080 1.080 1.079

Cα1 −Cβ1 Cα2 −Cβ2 O−C1 O−C2

1.359 1.358 1.366 1.363

1.371 1.370 1.364 1.360

Cα1 −Cα2 Cα2 −Cβ2 O−C1 O−C2

1.358 1.358 1.366 1.363

1.370 1.371 1.362 1.361


1.358 1.358 1.363 1.363

1.370 1.370 1.360 1.360

O−H1 O−H2 6 H O 2

0.962 0.962 105.3

0.960 0.960 103.6

O−H1 O−H2 6 H O 2

0.962 0.962 105.4

0.960 0.960 103.7

H2 O O−H1 O−H2 6 H O 2

0.962 0.962 105.1

0.960 0.960 103.5

C−H···O H2 O (tilt)

179.8 145.7

178.6 144.8

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C−H···O H2 O (tilt)

179.7 162.2

178.5 152.3

Point Group

CS

CS

Point Group

CS

CS

B3LYP 2.044 1.077 1.078 1.078 1.077

MP2 2.040 1.079 1.080 1.080 1.079

IV CH···C Cα1 −H Cβ1 −H Cβ2 −H Cα2 −H

B3LYP 2.517 1.077 1.079 1.078 1.077

MP2 2.522 1.079 1.081 1.080 1.079


1.356 1.356 1.368 1.368

1.368 1.368 1.365 1.365


1.360 1.358 1.362 1.363

1.373 1.371 1.360 1.359

O−H1 O−H2 6 H O 2

0.966 0.962 104.9

0.963 0.959 103.1

O−H1 O−H2 6 H O 2

0.966 0.962 104.9

0.963 0.960 102.5

173.5 177.1

170.6 167.4

6

167.3 87.8

166.8 75.6

CS

CS

C1

C1

6 6

III OH···O Cα1 −H Cβ1 −H Cβ2 −H Cα2 −H

6 6

CH···O κO···H

Point Group

6

6

CH···O κC···H

Point Group

a

All bond lengths are in angstroms and all angles are in degrees. For labeling purposes, α carbons indicate the carbons closest to the oxygen, whereas β indicates the farther carbons. The 1 and 2 designations indicate if the carbon is on the same side (1) as the complexing water or on the opposing side (2) as indicated in Figure 1. κ - centroid of the furan ring. b

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The geometric parameters of these structures are of note in several places. As may be expected, complex III shows the shortest interaction length of the four structures. In the B3LYP calculated structure, the monomers are separated by 2.044 ˚ A in complex III, whereas complex I and II show separations of 2.297 and 2.360 ˚ A, respectively. Complex IV is less well defined as there is no simple point of reference in this structure. However, the distance from the water monomer to the plane of the furan ring (collinear to the OH bond) was found to be 2.555 ˚ A, while the distance to the nearest carbon atom was found to be 2.517 ˚ A. Complexes I and II align well along the respective C−H bond of the furan monomer with each complex being within half degree of collinearity. Complex III shows a larger tilt, being roughly 6.5◦ from linear, with respect to the plane of the furan ring. The geometry of the water monomer shows modulation according to the complex that is formed. A lengthening of the OH bond is observed in all four structures, but to varying degrees. Complexes I and II show only minor changes and lengthen by 0.040 % (0.0004 ˚ A). Complex III shows a more significant change in the O−H bond, which lengthens by 0.44 % (0.0042 ˚ A), while Complex IV lengthens by 0.36 % (0.0034 ˚ A). Additionally, the bond angle of water is observed to change upon complexation. Complexes I and II show a small opening of the bond angle (0.21 and 0.32 ◦ , respectively), whereas complex III and IV show a narrowing of the bond angle (-0.19 and -0.20◦ , respectively). As noted earlier, a microwave study 29 indicated a complex geometry for Fu:H2 O that was nearly planar and showed large amplitude motions. This description is consistent with structures I, II, and III as they were calculated to be heavy-atom planar geometries. The structures each show one interaction between the two monomers, thus allowing for some freedom of motion. Structure IV was the only structure found to not show the oxygen atom of water in the same plane as furan, thus this structure would seem less likely to be that found in the microwave study. 29 Structures I and II have both protons of the water monomer out-of-plane, whereas structure III only has one proton out-of-plane and the second involved with a hydrogen bond interaction with the oxygen of furan. Unfortunately, the experimental

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rotational constants were not reported in the microwave study and therefore we cannot use that information to aid our analysis.

Energetics Multiple levels of theory (B3LYP, B3LYP-GD3BJ, M05-2X, and MP2) predict the lowest energy complex to be structure III (Table 2). Complex IV was found to be the second lowest, generally being over 1.6 kJ/mol higher in energy than complex III. Complex I and II are predicted to be the highest energy; always higher in energy by at least 2.8 kJ/mol with respect to structure III (all corrected for ZPE and BSSE). ω-B97XD, alone, predicts structure IV to be the lowest energy, but by only 0.2 kJ/mol with respect to structure III. The other two complexes (I and II) were found to be over 3.8 kJ/mol higher in energy, according to ω-B97XD. In all cases, the calculated interaction energy was found to be greatest (most negative) for complex III, followed by structure IV. This would make complex III seem to be the most likely to be observed in experiments.

Vibrational Frequencies The calculated, harmonic frequencies of the four complexes were expected to show diagnostic perturbation when compared to monomers. For simplicity, only the B3LYP calculated vibrational frequency shifts of interest to this study are presented (Table 3). As might be expected, significant perturbations were observed in the vibrational frequencies associated with the water monomer. In general, complexes I and II were found to have smaller shifts when compared to complexes III and IV (Figure 2). This is not surprising as complexes I and II show water acting as an H-bond acceptor whereas complexes III and IV have water acting as the H-bond donor. For simplicity, we will refer to the vibrational modes of the water monomers using standard Mulliken designations (ν1 is the symmetric stretch, ν2 is the bend, and ν3 is the asymmetric stretch). The vibrational modes for complexes III and IV will also be described as bound or unbound stretches, with the bound stretch being essentially

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Table 2: Calculated, relative energies, and interaction energies of Fu:H2 O complexes.

a

Structure B3LYP/6-311++G(d,p) Rel. Energy, ZPE corr. (kJ/mol) Rel. Energy, ZPE and BSSE corr. (kJ/mol) E(Complex, corr)a (kJ/mol)

I

II

3.00 4.26 0.00 3.57 4.55 0.00 -6.95 -5.27 -12.01

B3LYP-GD3BJ/6-311++G(d,p) Rel. Energy, ZPE corr. (kJ/mol) Rel. Energy, ZPE and BSSE corr. (kJ/mol) E(Complex, corr) (kJ/mol)

4.72 6.19 0.00 1.11 5.26 6.51 0.00 1.67 -9.58 -7.70 -16.40 -13.51

M05-2X/6-311++G(d,p) Rel. Energy, ZPE corr. (kJ/mol) Rel. Energy, ZPE and BSSE corr. (kJ/mol) E(Complex, corr) kJ/mol

5.61 7.00 0.00 0.79 6.21 7.29 0.00 1.70 -9.50 -7.78 -17.11 -13.64

ω-B97XD/6-311++G(d,p) Rel. Energy, ZPE corr. (kJ/mol) Rel. Energy, ZPE and BSSE corr. (kJ/mol) E(Complex, corr) (kJ/mol)

3.52 4.71 0.00 -0.82 3.83 4.86 0.00 -0.24 -9.16 -7.20 -14.31 -13.89

MP2/6-311++G(d,p) Rel. Energy, ZPE corr. (kJ/mol) Rel. Energy, ZPE and BSSE corr. (kJ/mol) E(Complex, corr) (kJ/mol)

2.78 4.31 0.00 2.83 4.09 0.00 -7.20 -5.61 -11.13

Calculated interaction energy of the complex, corrected for BSSE.

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III

IV 3.02 2.66 -7.45

0.45 1.96 -9.59

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ν1 and the unbound is ν3 . In complexes III and IV, the water ν1 (the bound O−H) redshifts by over 40 cm−1 , whereas complex I and II are negligibly redshifted (∼ 1 cm−1 ). The ν3 mode of water (unbound OH stretch) also shows a shift for complexes III and IV of roughly 25 cm−1 , which is a larger shift than observed for complexes I and II (< 3.1 cm−1 ). The water bend shows less connection to the H-bond motif, as complex III has the largest shift (+17.3 cm−1 ), complexes I and II have similar shifts (+13.6 and +13.2 cm−1 , respectively), and complex IV shows the smallest shift (8.5 cm−1 ). From these vibrational shifts, it seems likely that unambiguous assignments of complex geometry may be possible. Several vibrational modes of furan were also calculated to shift upon complexation, however these modes appear to be less diagnostic than the water modes. The ν13 band is predicted to be the strongest absorption feature in furan and is calculated to be found at 755.1 cm−1 . Upon complexation, shifts of 34.1, 13.6, 1.0, and 10.1 cm−1 were calculated for complexes I - IV, respectively. This mode shows the greatest potential for diagnostic behavior (of the furan vibrations). Most other furan modes show only modest shifts (< 5 cm−1 ), show similar shifts of all four complexes, or are only diagnostic with respect to one complex. For example, ν19 is calculated to shift by -2.3, 1.5, -6.1, and -0.9 cm−1 (for complexes I - IV, respectively), thus it would seem that only the shifted ν19 for complex III could be easily resolved in FTIR studies. While the current experimental setup only allows for detection from 4000 - 600 cm−1 in the IR, it is recognized that the low-frequency, intermolecular vibrations can be quite diagnostic to interactions of a complex. Indeed Fu:H2 O, has several low-frequency modes that could serve as diagnostic vibrations in a future study (Table 3). In fact, the calculated intensity of at least one mode in each complex is greater than the highest intensity mode in furan. In complexes I and II, the strongest vibration is a water rocking motion (calculated 166.7 and 78 cm−1 , respectively), whereas complex III and IV show a water torsion motion (calculated at 104.9 and 68.4 cm−1 , respectively.) Additionally, complex III has an intermolecular vibration at 488.1 cm−1 , far removed from all other the complexes or any intramolecular

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Table 3: Calculated vibration frequency shifts for Fu:H2 O, isotopologues, and intermolecular vibrations at the B3LYP level of theory.a B3LYP Mode ν13 ν8 ν7 ν6 ν19 ν3 ν17

Monomer Freq. 746.0 876.7 999.6 1070.1 1181.0 1487.8 1587.9

I ∆b 34.1 0.0 -1.5 -0.9 -2.3 3.5 -0.6

II ∆ 13.6 0.7 6.8 1.2 1.5 -1.2 1.0

III ∆ 1.0 4.5 -3.1 -9.6 -6.1 4.1 7.3

IV ∆ 10.1 0.2 0.3 0.0 -0.9 -1.3 -4.1

CH CH CH CH

3111.0 3121.3 3145.3 3151.3

-3.3 -4.1 -13.7 -4.9

-6.5 -5.5 -2.9 -3.3

2.6 2.3 3.0 2.6

0.7 0.8 0.5 0.4

H2 O

ν2 ν1 ν3

1583.6 3664.5 3765.4

13.6 -1.2 -3.1

13.2 -0.8 -1.4

17.3 -42.9 -24.6

8.5 -40.4 -24.8

D2 O

ν2 ν1 ν3

1160.0 2641.4 2759.5

9.8 -0.8 -2.1

10.1 -0.6 -0.7

11.7 -26.9 -21.7

5.0 -25.6 -21.2

DOHc

bend O−D O−H

1388.0 2698.6 3716.9

12.6 -1.5 -2.1

12.8 -0.7 -1.0

-0.8 -51.7 4.1

-2.0 -47.9 1.4

HODc

bend O−D O−H

1388.0 2698.6 3716.9

12.6 -1.4 -2.1

12.8 -0.8 -1.0

32.3 3.7 -72.2

17.6 1.5 -67.0

Intensity 8.4 1.6 106.9 0.7 81.4 87.3

IV Freq. 20.2 32.0 68.4 90.1 174.9 290.3

Furan

IM1 e IM2 IM3 IM4 IM5 IM6

I Freq. 33.6 46.7 52.8 93.4 166.7 195.8

Intensityd 11.1 30.2 4.5 2.0 231.8 35.2

II Freq. 32.3 42.3 69.2 78.8 83.8 169.9

Intensity 5.1 17.1 14.2 375.6 1.9 77.5

III Freq. 20.1 31.1 104.9 116.7 202.2 488.1

a

Intensity 3.6 2.2 96.8 3.6 88.5 71.3

OH stretches were scaled by 0.960 and all other vibrations have been scaled by 0.988. ∆ = νcomplex − νmonomer c DOH indicates interaction of complexes III and IV is through D, whereas HOD interacts through H. d Intensities are normalized to the ν13 mode of furan in its respective complex. The intensity of the ν13 band was set arbitrarily to 100. e Intermolecular vibration

b

12


a.)

Mono I II III IV

Intensity Calculated Intensity

H2O

III

I

Calculated Intensity

I


II

b.)

II


Mono I II III IV

D2O

Mono I II III IV

IV

3800 3800

3750 3750

3700 3700

3650 3650

1650 1600 1550 1550 1650 -1 Wavenumbers (cm )

3600 3600

2800 2800

-1 Wavenumbers (cm -1)

Wavenumbers (cm )

c.)

Mono I II III IV

III

IV

2750 2750

2700 2700

2650 2650

1150 1200 1150 1200 -1 (cm )

2600 2600

-1 -1 ) Wavenumbers (cm Wavenumbers

Wavenumbers (cm )

II

Mono I III H IV H

III

*

IV

*

II III D IV D

Mono I III H IV H

II III D IV D

*

Absorbance

HOD

Absorbance


I

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity

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Mono I III H IV H

II III D IV D

*

* 3800 3800

3750 3750

3700 3700

3650 3650 -1

Wavenumbers (cm )

3600 3600

2720 2720

2680 2680

2640 2640

Wavenumbers (cm-1-1)

Wavenumbers (cm )

* 2600 2600

1420 1420

1400 1400

1380 1380 -1

Wavenumbers (cm )

Figure 2: Calculated (B3LYP) vibrational frequencies for the four minimum energy Fu:H2 O complexes. Panel a.) shows vibrations of H2 O, b.) shows D2 O vibrations, and c.) shows HOD vibrations. For HOD complexes III and IV, starred transition indicate when the monomer is bound through D and unmarked transitions are bound through H. The dotted line was added to show the relative position of the monomer. The spectra have been offset for clarity. Calculated intensities are in km/mol. vibrations. While these vibrational features cannot be observed in our experiments, they could be useful in future studies of this complex in other spectral regions.

Electron Topology of the Complexes AIM analysis was used to further characterize the observed interactions of Fu:H2 O. Koch and Popelier proposed several criteria to validate the hydrogen bonding interactions, including the existence of a bond critical point (BCP) between the donor and acceptor atoms with 13



electron density (ρ) and the Laplacian of the charge density (∇2 ρ) values at this point being within reasonable ranges (0.024 to 0.139 au). 46 AIM analysis shows all calculated structures of Fu:H2 O to have at least one BCP between suspected donor and acceptor atoms. (Figure 3). Only ω-B97XD calculations show a structure (found in the Supporting Information) that contains more than one BCP between the two monomers. In this structure, two BCPs are observed as both hydrogens of the water molecule interact with the π cloud of furan. It should be noted that this level of theory determined a structure that differed from all other levels of theory as it is nearly CS as opposed to the C1 structure indicated in Figure 1. This double donor structure also shows a ring critical point (RCP) within the interaction ring of the furan and water.

Figure 3: AIM plots of the calculated structures of Fu:H2 O. All structures were optimized using B3LYP/6-311++G(d,p). Bond critical points are shown in green and ring critical points are shown as small, red spheres. The dotted line represents the electron density paths. In all four calculated structures, the ρ values were small (0.0101 - 0.187 a.u.) and the Laplacian charge densities (∇2 ρ) were small and positive (0.0263 - 0.0747 a.u.) (Table 4). These values are well inline with other van der Waals or weak hydrogen bond interactions. 30,32,46 Complex III shows the largest ρ and ∇2 ρ values of the four calculated complexes, 14


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whereas complex IV shows the smallest values. This ordering is consistent from B3LYP to MP2 calculations, but small absolute differences can be observed, indicating general agreement between the calculated electron topographies across levels of theory. The electron density values (Table 4) calculated here can be compared to other weaklybound systems. The Fu:H2 O complexes could be viewed as having similar degrees of interaction as H2 S dimer, PH3 :H2 O, and C2 H4 :HF, but significantly weaker than H2 O dimer or PH3 :HF. 47 Additionally, M05-2X calculations show ρ values for benzene:water that are comparable in magnitude (0.0072 and 0.0121 a.u.) for the two OH···π interacting structures. 14 Thus, it would be reasonable to think of the furan : water interaction as a weak hydrogen bond without major deviations from similar systems. Table 4: AIM properties for the B3LYP and MP2 calculated structures of Fu:H2 O. B3LYP Structure Interaction I C−H···O II C−H···O III O−H···O IV O−H···π

ρ (a.u.) ∇2 ρ (a.u.) 0.0119 0.0410 0.0104 0.0357 0.0186 0.0726 0.0098 0.0263

MP2 Structure Interaction I C−H···O II C−H···O III O−H···O IV O−H···π

ρ (a.u.) ∇2 ρ (a.u.) 0.0122 0.0431 0.0111 0.0388 0.0187 0.0747 0.0101 0.0305

FTIR Spectra H2 O transitions FTIR spectra were acquired of each furan, water, and mixtures of the two, in order to observe potential Fu:H2 O complexes. In all spectra, there are many small peaks in the OH stretch and bend regions that can be ascribed to incomplete subtraction of atmospheric H2 O. In the spectrum of the 5:1:5000 Fu:H2 O:N2 , bands of each monomer were readily apparent. The 15



O−H stretches of monomeric H2 O were found at 3727.4 (ν3 ) and 3634.9 cm−1 , in agreement with the literature (Figure 4). 21,48–51 As deposited, there were a few potential new bands in this region, but they were very weak. These bands grew in intensity upon annealing. The newly observed bands do not appear in annealed spectra of pure water under the same conditions nor furan, thus they likely arise from a combination of the two species (Figure 5). This indicates the new transitions may be due to weakly bound complexes, as annealing is often found to increase the intensity of transition due to complexes. 21,32,52 As complexes are the focus of this study, only annealed spectra will be used.

s_1212_05 s_1212_06 annealed

0.8 0.8

* Absorbance Normalized Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 32

0.6 0.6

*

*

*

b.)

0.4 0.4

0.2 0.2

a.) 0.0 0.0

3750 3750

3700 3700

3650 3600 3650 3600 -1 -1) Wavenumbers Wavenumbers (cm(cm )

3550 3550

Figure 4: Normalized FTIR spectrum of the OH stretch region of Fu:H2 O:N2 (5:1:5000) a.) as deposited at 15 K and b.) after annealing at 30 K for 15 minutes. The spectra have been offset for clarity. The growth of new bands (marked with *) upon annealing is a signature of complex formation in the matrix. A pair of transitions grew in and became quite apparent at -14.9 and -18.5 cm−1 with respect to ν3 of H2 O (Table 5). It would seem most likely these transitions represent the unbound OH stretch that is shifted upon complexation. A second pair were observed at -19.2 and -22.5 cm−1 with respect to ν1 of H2 O (Table 3) and were assigned as the bound OH stretch. There are three possible explanations for the paired look of these transitions: 1.) matrix site splittings, 2.) rotating water transitions, or 3.) multiple complex geometries are formed in the matrix. Matrix effects can often be verified by changing the matrix gas. 16


Page 17 of 32

1.5 1.5

2.5 2.5

i

1.0 1.0

c.)

*

*

Normalized Absorbance

* Normalized Absorbance


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


*

n3 (H2O)

0.5 0.5

n1 (H2O)

b.)

0.0 0.0

a.)

3750 3750

2.0 2.0

1.5 1.5

3650 3650

c.)

*

W *

?

n2 (H2O) 1.0 1.0 b.)

0.5 0.5 0.0 0.0

3700 3700

ii

3600 3600

-1 ) (cm-1

n17 (Fu) a.) 1620 1620

1610 1610

1600 1600

1590 1590

1580 1580

-1 -1 ) (cm

Wavenumbers Wavenumbers (cm )

Wavenumbers (cm ) Wavenumbers

Figure 5: Normalized FTIR spectrum of the i.) OH stretch and ii.) OH bend regions, after annealing. The three traces represent a.) Fu:N2 (1:1000) b.) H2 O:N2 (1:5000), and c.) Fu:H2 O:N2 (5:1:5000) The spectra have been offset for clarity. Many of the smallest features in c.) are due to incomplete subtraction of atmospheric water lines. It should be noted that traces of H2 O were still found in furan samples even after repeated attempts of drying. Transitions of 1:1 complexes of Fu:H2 O are indicated with *, a water dimer band is marked with W, and another, unidentified complex band is marked with a ? mark. This is complicated for the system as H2 O is known to show rotational structure in some matrices (e.g. Ar) and not in others (N2 ). 51 Regardless, spectra of Fu:H2 O were acquired in an argon matrix (see the Supporting Information) and splittings of potential Fu:H2 O bands were not clear. This region is dense and has poor signal to noise after annealing, thus we are hesitant to claim there are or are not splittings in this region. Thus, it is unclear if the pairs of transitions observed in the nitrogen matrix are indicative of matrix effects. Spectra were also recorded at 10 K (see the Supporting Information) to probe for potential rotating water bands. There were few differences observed at this lower temperature and all changes were minor in nature. This would argue against the existence of rotating water transitions. The calculated shifts for complexes III and IV are in reasonable agreement with the observed shifts in this region, thus it is also plausible that multiple geometries exist in the matrix.

17



Given the subsequent analysis, we chose to assign this feature to matrix site splittings. In the H2 O bend region, a complex band was identified at 1618.1 cm−1 (+20.8 cm−1 ). This band is consistent with complex III which is calculated to be +17.3 cm−1 . This band is near degenerate with a (H2 O)2 transition, 48–50 which complicated initial assignment. Closer inspection of this band showed a small, but reliable shift from the H2 O dimer position. There do not appear to be any bands that obviously correlate to complexes I and II (calculated +13.6 and +13.2 cm−1 , respectively) in this region. A small band was observed at +2.5 cm−1 and could be related to complex IV (calculated +8.5 cm−1 ), but this band does not track appropriately with changes in furan concentrations within the matrix. This band is also at the same position as the ν2 , proton acceptor, transition of H2 O dimer. Thus, it seems unlikely that this band originates from a 1:1 complex of Fu:H2 O. Another complex band is observed in this region at 1589.6 cm−1 , but this band does not connect to any shift of the water monomer. Possible origins of this band include complex transitions from a furan band or higher order cluster. As there were no close bands of furan (with respect to calculated shifts) it seems more likely that this band is from a higher order cluster. None of the potential complex transitions show signs of matrix site splitting. Again, potential transitions of the complex only appear when both monomers are present in the matrix.

Furan transitions There are some indications of complex bands associated with furan vibrations, but as was noted earlier, they are less obvious and diagnostic. As a result, we focus on the most useful band found in our spectrum, ν13 . The ν13 band of furan is the strongest transition in the spectrum, is found near 750 cm−1 , and appears to be split into three components (Figure 6). The reason for this splitting is unclear. A similar cluster of peaks is observed when furan is deposited in argon (see the Supporting Information), arguing against multiple matrix sites. No new bands grow in when the two monomers are in the matrix together, nor do the relative intensities of the triad show a strong correlation to the water content of the sample.

18


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Table 5: Experimental FTIR frequencies of H2 O monomer and associated transitions for the Fu:H2 O complex and isotopologues in a N2 matrix. Fu:H2 O Frequency (cm ) Shift (cm−1 ) 3727.4 — 3712.5 -14.9 3708.9 -18.5 −1

Assignment ν3 (H2 O) Fu:H2 O Fu:H2 O

3634.9 3615.7 3612.4

— -19.2 -22.5

ν1 (H2 O) Fu:H2 O Fu:H2 O

1597.3 1618.1

— 20.8

ν2 (H2 O) Fu:H2 O

Fu:D2 O Frequency (cm ) Shift (cm−1 ) 2765.9 — 2752.5 -13.4 2749.6 -16.3 −1

Assignment ν3 (D2 O) Fu:D2 O Fu:D2 O

2655.5 2642.5 2640.4

— -13.0 -15.1

ν1 (D2 O) Fu:D2 O Fu:D2 O

1178.7 1193.2

— 14.5

ν2 (D2 O) Fu:D2 O

Fu:HOD Frequency (cm ) Shift (cm−1 ) 3681.8 — 3684.4 2.6 3611.0 -70.7 −1

Assignment OH str. (HOD) Fu:HOD (D bound) Fu:HOD (H bound)

2705.8 2709.3 2647.9 2672.3

— 3.5 -30.9 -33.5

OD str. (HOD) Fu:HOD (H bound) Fu:HOD (D bound) Fu:HOD (D bound)

1404.7 1415.5

— 10.8

bend (HOD) Fu:HOD

19



The calculated frequency for ν13 of complex III shows very little perturbation (1.0 cm−1 ) upon complexation, whereas the other three show blue shifts of greater than 10 cm−1 (Table 3). Given ν13 is the strongest vibrational transition in furan, it would stand to reason that if shifted bands were present, they should be plainly observed. However, if the shift due to complexation is on the order of the instrumental resolution, a change may not be readily apparent. In the spectral region of ν13 , there is no indication of shifted transitions (Figure 6). In order to better show this lack of structure, the difference spectrum of Fu:H2 O:N2 − Fu:N2 − H2 O:N2 is shown (Figure 6). Here, only two small bands are observed, which are nearly degenerate with the furan monomer lines. This would indicate there are no significantly shifted bands in this region. This lack of change would argue for the formation of complex III alone. The aforementioned vibrational frequency calculations also indicated the ν19 (monomer calculated at 1181.0 cm−1 ) to potentially be diagnostic, but inspection of this region showed no clearly resolved bands that could be ascribed to complex formation. 4

Absorbance

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 32

a.)

0

b.)

-2 diff1212_06 s_0121_09c

-4 770 770

760 760

750 740 740 750 -1 Wavenumbers Wavenumbers (cm(cm ) -1)

730 730

Figure 6: Normalized FTIR spectrum of the ν13 region of furan, after annealing. The two traces represent a.) Fu:H2 O:N2 (5:1:5000) and b.) the difference spectrum (Fu:H2 O:N2 −Fu:N2 −H2 O:N2 ) to better show bands that relate to complex formation. The largest peak in this region was cut off to better show the low intensity structure in this region. As the C−H stretch region of furan shows some calculated differentiation among the four 20


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complexes, it might seem this could be a diagnostic region of the spectrum. Unfortunately, the annealed spectra show complex behavior and only minor perturbations in this region making unambiguous assignments challenging (see the Supporting Information). This region is further complicated as the H atoms of furan will likely be interacting with the local matrix leading to additional splittings. Due to these difficulties, arguments based on C−H stretches will not be presented.

D2 O and HOD spectra To aid in the clarity of this analysis, the spectra of complexes with water isotopologues (D2 O and HOD) were acquired and the corresponding vibrational frequencies were calculated (Figure 7, Table 3). The spectra of these species show strong bands at 2765.8, 2655.4, and 1178.7 cm−1 corresponding to D2 O monomer absorbance and bands at 3681.8, 2705.7, and 1404.7 cm−1 corresponding to HOD monomer. Upon annealing, new bands appear around the D2 O and HOD monomer transitions, similar to those observed around H2 O monomer bands. Transitions corresponding to shifted OD stretches (for D2 O) were observed at 2642.5, 2749.6, 2642.5, and 2640.4 cm−1 . These transitions show similar site splittings as were seen in Fu:H2 O bands. Bands corresponding to shifted OH stretches of Fu:HOD were found at 3684.4 and 3611.0 cm−1 , whereas the shifted OD stretches were identified at 2709.3, 2647.9, and 2672.3 cm−1 . There appear to be more bands for the OD stretches than OH as the OD stretches show matrix site splittings wheres the OH stretches do not. The reason for this difference is not readily understood. Interestingly, the band at 3684.3 cm−1 , was blue-shifted from the HOD, OH stretch by 2.5 cm−1 (Table 3). In the calculated frequencies, only complex III indicated a significant blue-shift of this mode. This vibration is specifically in the case where the HOD species interacts with furan through the deuterium. In other words, we observe a blue-shift in the unbound atom stretch. There was also a corresponding blue-shift predicted in the OD stretch, for the isomer interacting through the H atom, but the intensity was calculated to

21



be weak. Indeed, there is a small band, blue-shifted by 3.5 cm−1 (3.9 cm−1 calculated). This would seem to be clear evidence for the existence of complex III in the matrix. As seen in Table 3, complex IV also shows a blue-shift of these unbound stretches, but they are subtle (< 2 cm−1 ) and would not be expected to be readily resolved spectral features. The bend regions of D2 O and HOD were less complicated than the bend region of H2 O. In both isotopologue regions, a single, blue-shifted transition was assigned to the complex (Figure 7). The D2 O bend for the complex was observed at 1193.2 cm−1 (shift of 14.5 cm−1 from the monomer) whereas the HOD bend of the complex was found at 1415.5 cm−1 (shifted by 10.8 cm−1 from HOD monomer). The observed D2 O bend is consistent with complexes I - III as each are shifted by nearly the same amount (calculated 9.8 - 11.7 cm−1 ). The HOD bend region matched best with complexes I and II (calculated 12.6 and 12.8 cm−1 , respectively) whereas III and IV are shifted significantly more (calculated 32.3 and 17.6 cm−1 , respectively). This is the only occurrence where a calculated vibration seems to match more appropriately to complexes I and II than III and IV.

Assignment of a complex geometry When the data is analyzed holistically, a general consensus to the complex geometry can be obtained. The geometric optimizations point to complex III being the overall most stable structure (Table 2) and therefore most likely to be observed experimentally. Complex IV was generally seen close in energy to complex III and could potentially be observed. Complexes I and II were consistently the highest energy and thus seem the least likely to be observed. The calculated stabilization energy is also greatest (most negative) for complex III and the electron density analysis showed the largest density in complex III. Thus all of the computational data point to complex III being the most likely formed in the matrix experiment. Vibrational calculations and measurements support complex III as the only structure formed in the matrix. The split bands observed in Fu:H2 O at 14.9/18.5 and -19.2/22.5 cm−1 22


Page 22 of 32

Page 23 of 32

i

0.25

0.5 0.5 c.)

*

*

*

0.4 0.4 n3 (H2O)

0.3 0.3 0.2 0.2

** DO–H

n1 (H2O)

b.)

* * *

c.)

0.15

* * HO–D

n3 (D2O)

0.10 n1 (D2O)

b.)

0.05

0.1 0.1 a.)

a.)

0.0 0.0 3680 3640 3720 3720 3680 3640 -1-1 Wavenumbers (cm Wavenumbers (cm ))

0.5 0.5

0.00 2800 2800

0.15 0.15

*

c.)

c.) H-O-D

b.)

0.2 0.2 0.1 0.1

0.00

a.) 1420 1400 1420 1400 -1 Wavenumbers (cm Wavenumbers (cm-1) )

1380 1380

*

n2 (D2O)

0.10

0.05

n4 (Fu)

0.0 0.0 1440 1440

2760 2720 2680 -1 Wavenumbers(cm (cm-1)) Wavenumbers

iv

iii

0.4 0.4

0.3 0.3

3600 3600

Absorbance

0.6 0.6


ii

0.20

Absorbance


Absorbance

0.6 0.6

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


n19 (Fu)

b.) a.)

1200 1200

1170 1160 1180 1190 1190 1180 1170 1160 -1-1 Wavenumbers (cm ) Wavenumbers (cm )

Figure 7: Normalized FTIR spectra of the i.) OH stretch region of HOD, ii.) OD stretch region of D2 O, iii.) HOD bend region, and iv.) D2 O bend region for Fu:D2 O. In each panel, a.) is Fu:N2 (1:3000), b.) is D2 O:N2 (1:3000), and c.) is Fu:D2 O:N2 (1:1:3000). Transitions assigned to 1:1 complexes are identified with *. There is a significant amount of H2 O observed as an impurity in the spectra, thus H2 O and HOD are readily observed. The full intensity of the ν19 transition of furan was cut of to better show the lower intensity structure of the D2 O bend.

23


1150 1150


correlate well with the calculated unbound and unbound OH vibrations, respectively, for complex III (calculated -24.6 and -46.9 cm−1 ) and complex IV (-24.8 and -40.4 cm−1 ). The stretches for complexes I and II do not shift enough in this region to unequivocally preclude these structures. The vibrational shifts of the OD stretches in Fu:D2 O (measured -13.4/-16.3 and -13.0/15.1 cm−1 , calculated -21.7 and -26.9 cm−1 , respectively) and the H2 O (measured 20.8 cm−1 , calculated 17.3 cm−1 ) and the D2 O (measured 14.5 cm−1 , calculated 11.7 cm−1 ) bends are also consistent with the assignment of complex III. In HOD complexes, a characteristic blue shift was observed in matrix experiments that best matches the calculated behavior of complex III alone. The furan ν13 is helpful as vibrational frequencies were predicted to show sizable shifts for all complexes, except complex III. As no bands were observed to shift in this region, we can conclude complex III is the lone geometry observed in the matrix. While the HOD bend region seems to better match with complexes I and II, the remainder of the spectroscopic and computational data point to complex III alone. This oddity could be due to the harmonic nature of the vibrational frequency calculations. These results are also consistent with the aforementioned microwave study 29 in that complex III is mostly planar and could certainly show large amplitude motions from the free OH. This geometry is also consistent with previous results on dimethylfuran : methanol 44 and furan : methanol complexes, 30 which both indicated complexes stabilized by O−H···O interactions. Given the overwhelming data (spectroscopy, computational, and related studies), we are confident in assigning an O−H···O interacting geometry to Fu:H2 O, such as that given in III.

Conclusions The FTIR spectrum of the furan : water complex was recorded in cryogenic matrices at 15 K. Analysis of these spectra indicated the formation of a complex that led to a substantial shift of the OH stretches of the water monomer. Analogous shifts were also observed when

24


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complexes were formed with water isotopologues. Interestingly, a blue-shift of the unbound OH(OD) stretch was observed for HOD complexes in both experiment and calculations. This blue-shift was determined to be diagnostic for complex III, which is characterized by a single O−H···O interaction. The ν13 vibration of furan was also found to be diagnostic as it showed no sign of complex vibrational bands. This lack of structure indicated that no other geometries were likely formed in the matrix.

Acknowledgement The authors gratefully acknowledge support of Hobart and William Smith Colleges for laboratory startup funding. Computational resources of the Extreme Science and Engineering Discovery Environment (XSEDE), a program supported by National Science Foundation grant number OCI-1053575, were used in this work.

Supporting Information Available Supporting information available. This includes the full citation for reference 34, Cartesian coordinates for the B3LYP/6-311++G(d,p) structures, the AIM analysis structure for complex IV as calculated using ω-B97XD, matrix isolation FTIR spectra of the CH stretch region of the Fu:H2 O:N2 spectrum, temperature effects on the Fu:H2 O:N2 spectrum, and selected regions of the Fu:H2 O:Ar spectrum.

This material is available free of charge via

the Internet at http://pubs.acs.org/.

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Bonded Dimers. HCN· · ·HF as the Spectroscopic Prototype. Chemical Reviews 1986, 86, 635–657. (3) Zwier, T. S. The Spectroscopy of Solvation in Hydrogen-Bonded Aromatic Clusters. Annual Review of Physical Chemistry 1996, 47, 205–241. (4) Ahn, D.-S.; Jeon, I.-S.; Jang, S.-H.; Park, S.-W.; Lee, S.-Y.; Won-Jo;, W.-J. Hydrogen Bonding in Aromatic Alcohol-Water Clusters: A Brief Review. Bulletin of the Korean Chemical Society 2003, 24, 1336–1350. (5) Becucci, M.; Pietraperzia, G.; Pasquini, M.; Piani, G.; Zoppi, A.; Chelli, R.; Castellucci, E.; Demtroeder, W. A Study on the Anisole-Water Complex by Molecular BeamElectronic Spectroscopy and Molecular Mechanics Calculations. Journal of Chemical Physics 2004, 120, 5601–5607. (6) Desfran¸cois, C.; Carles, S.; Schermann, J. P. Weakly Bound Clusters of Biological Interest. Journal of Chemical Physics 1989, 90, 1007–1023. (7) Scheiner, S.; Kar, T.; Pattanayak, J. Comparison of Various Types of Hydrogen Bonds Involving Aromatic Amino Acids. Journal of the American Chemical Society 1989, 124, 13257–13264. (8) Murray, C.; Derro, E. L.; Sechler, T. D.; Lester, M. I. Weakly Bound Molecules in the Atmosphere: A Case Study of HOOO. Accounts of Chemical Research 2009, 42, 419–427. (9) Autumn, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.; Sponberg, S.; Kenny, T. W.; Fearing, R.; Israelachvili, J. N.; Full, R. J. Evidence for van der Waals Adhesion in Gecko Setae. Proceedings of the National Academy of Sciences 2002, 99, 12252–12256.

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31



Graphical TOC Entry 1.4


ν3 (H2O) 1.2

s_0121_09c s_1213_04c s_1212_06c

FU:N2 H2O:N2 H2O:FU:N2

*

ν1 (H2O)

0.8

0.6

1:1000 1:5000 1:5:5000

* Furan:H2O

1.0 Normalized Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

* Furan:H2O *

3750

3700 3650 Wavenumbers (cm-1)

3600

3750

3700

3600

0.4

0.2

0.0 3650 -1

Wavenumbers (cm )

32


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Structure and Spectroscopy of Furan:H2O Complexes - The Journal of

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