Molecularly Tuning the Radicaloid NâHÂ·Â·Â·O C Hydrogen Bond - The

Article pubs.acs.org/JPCA

Molecularly Tuning the Radicaloid N−H···OC Hydrogen Bond Norman Lu,*,†,‡ Wei-Cheng Chung,‡ Rebecca M. Ley,† Kwan-Yu Lin,‡ Joseph S. Francisco,*,†,§ and Ei-ichi Negishi*,† †

Department of Chemistry, Purdue University, 1393 Brown Building, West Lafayette, Indiana 47907-1393, United States Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei 106, Taiwan § Department of Chemistry, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States ‡

S Supporting Information *

ABSTRACT: Substituent effects on the open shell N−H···OC hydrogen-bond has never been reported. This study examines how 12 functional groups composed of electron donating groups (EDG), halogen atoms and electron withdrawing groups (EWG) affect the N−H···OC hydrogen-bond properties in a six-membered cyclic model system of OC(Y)−CHC(X)N−H. It is found that group effects on this open shell H-bonding system are significant and have predictive trends when X = H and Y is varied. When Y is an EDG, the N−H···OC hydrogen-bond is strengthened; and when Y is an EWG, the bond is weakened; whereas the variation in electronic properties of X group do not exhibit a significant impact upon the hydrogen bond strength. The structural impact of the stronger N−H···OC hydrogen-bond are (1) shorter H and O distance, r(H···O) and (2) a longer N−H bond length, r(NH). The stronger N−H···OC hydrogen-bond also acts to pull the H and O in toward one another which has an effect on the bond angles. Our findings show that there is a linear relationship between hydrogen-bond angle and N−H···OC hydrogen-bond energy in this unusual H-bonding system. In addition, there is a linear correlation of the r(H···O) and the hydrogen bond energy. A short r(H···O) distance corresponds to a large hydrogen bond energy when Y is varied. The observed trends and findings have been validated using three different methods (UB3LYP, M06-2X, and UMP2) with two different basis sets.

■

INTRODUCTION A hydrogen bond is defined as A−H···B, where A and B are the hydrogen bond donor and acceptor, respectively. A hydrogen bond is typically characterized by three changes in the A−H bond properties: (i) elongation of the A−H bond, (ii) red shift of the fundamental A−H stretching transition, and (iii) intensity enhancement of the transition.1 A strong hydrogen bond often involves highly electronegative elements such as the second row elements (N, O, and F).2 Hydrogen-bonded complexes with oxygen as the hydrogen bond acceptor have been investigated to a great extent and are well characterized.3 The bulk of these studies have focus on hydrogen bonding in closed-shell systems. Since hydrogen bonding governs many chemical and biological processes in nature4,5 and is a critical factor in determining the physical properties of many solvated species in aqueous systems,6 “typical” hydrogen bonds in these processes are usually in the range of 1−7 kcal/mol,4,5 as is exemplified by the water dimer H2O−H2O, which has a binding energy ca. 4.9 kcal/mol; and is considered the classical prototype of closed-shell hydrogen bonding. However, a growing literature has focused on stronger hydrogen bonds, which appear to influence the structure and function of proteins and DNA.7,8 Much stronger hydrogen bonds are also known to exist in anionic systems.9,10 Strong hydrogen bonding has also been observed in concentrated aqueous solutions of strong acids,11 suggesting that a wider range of hydrogen bonding strengths is also possible for neutral, closed-shell systems. Open-shell hydrogen-bonded complexes, on the other hand, have only recently become subjects of significant experimental © XXXX American Chemical Society

and theoretical studies, presenting new challenges for our present understanding of hydrogen bonding.12 Much of our current knowledge of these systems derives from studies of the hydroperoxy (HO2) radical, whose hydrogen-bonded complexes with closed-shell species are important components of many biological, atmospheric, and combustion processes.4−7 A conspicuous feature of open-shell complexes formed with the HO2 radical is that their binding energies appear to be systematically larger than those of the water dimer or other analogous H2O−molecule complexes.13,14 Strongly bound open-shell HO2−molecular complexes are now known for nitrous, nitric, peroxynitric, formic, and acetic acids.14,15 Strong hydrogen bonding interactions in HO2−molecule complexes are expected to correlate with the enhanced acidity of HO2. Indeed, the gas-phase deprotonation energy for HO2 was calculated16 and found to be lower than that of H2O. The presumed relationship between gas-phase acidity and hydrogenbonding strength is also consistent with the experimental finding of Brauman and co-workers6 that the enhancement of the acidities and electron affinities of dithiols, compared to analogous monothiols, are attributable to extra stabilization of the intramolecular hydrogen bond in these systems. They also find that the hydrogen bonding in these systems has of cyclic pattern, a tendency that is also found for strongly bound openshell complexes of HO2. However, the studies of Brauman and Received: January 6, 2016 Revised: February 2, 2016

A

DOI: 10.1021/acs.jpca.6b00144 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A co-workers6,17,18 focused on anionic systems, whereas the focus of this work is on neutral radical-molecule systems with intermolecular hydrogen bonds. Studies of radical-molecule complexes have led to a general finding of unusually strong hydrogen bonding for neutral systems. Li, Alkorta, and others have studied hydrogen bonding in open shell H-bonding systems. Several interesting studies19−25 have been reported for open shell systems. Leopold et al.26 have coined the phrase “partially bonded complexes” for systems that have unusually strong hydrogen bonds but also show characteristics of weak covalent bonding. This terminology is consistent with the general viewpoint26 that hydrogen bonding is primarily an exchange-type “chemical” or “resonance” phenomenon of partial covalent character, so that stronger hydrogen bonding inherently manifests the signatures of partial covalent bonding. The N−H···OC hydrogen bond has been extensively studies for closed-shell systems because of their importance in biological systems, as related to peptide and protein folding.4,5,27,28 We examine if the same characteristics that define the N−H···OC hydrogen bond in closed-shell systems apply to the N−H···OC hydrogen bond in open shell systems. In Figure 1a, the intramolecular open shell N−H···

using the UMP2/aug-cc-pVTZ method with the UB3LYP/augcc-pVTZ geometry were also performed. Ab initio calculations using the unrestricted second order Møller−Plesset perturbation theory were performed with all orbitals active. We have incorporated the UMP2 results in the text as well as in the Supporting Information. All potential energy surface scans were carried out at the UB3LYP/aug-cc-pVDZ level of theory to identify possible minima. The geometries from the UB3LYP/ aug-cc-pVDZ were used as initial starting geometries for the UB3LYP/aug-cc-pVTZ geometry optimizations. Full geometry optimizations were carried out on identified minima corresponding to the transoidal (non-hydrogen bonded) conformation. To verify the nature of the minima as stationary points, frequency calculations on the optimized geometries were performed at each level of theory. Charges and spin densities are used from the Mulliken population analysis. An analysis of the stabilizing natural bond orbital interactions representative of the hydrogen bond interaction were calculated for the appropriate species with the NBO 6.0 program30 using second order perturbation theory analysis with 0.1 kcal mol−1 thresholds for donor−acceptor interaction energies using the UB3LYP/aug-cc-pVDZ level of theory.

■

RESULTS AND DISCUSSION Mulliken and natural bond orbital analyses were used to investigate the nature of hydrogen bonding in a model system of an open-shell six-membered ring with N−H···OC interaction in a system of OC(Y)−CHCC(X)N−H [where X, Y  H, F, Cl, Br, OH, NH2, CN, NO2, C(O)H, CCH, CH2CH, or CH3] in order to elucidate the group effect on the strength of radical hydrogen bonds as shown in Figure 1. Results from this study suggest that the different substituents exert a strong influence on the strength of open shell hydrogen bonding for N−H···OC interaction. This interaction is found to contribute to the unusually strong binding in this six-membered radical-molecule analogue. These findings can shed new light on our fundamental understanding of substituent tuning of the N−H···OC open shell hydrogen bond. Several weak proton donors and proton acceptors, whose interaction energies are within a few kcal/mol of each other, have been previously studied in applications of nanotechnology and supramolecular chemistry. Among these, the N−H···F, C− H···O, C−H···N, and C−H···π, C−H···F−C interactions3,31−34 appear to be most important in solvation processes. In view of the importance of this weak hydrogen-bonding interaction in material science, nanotechnology and biological systems,35,36 the model shown in Figure 1a which displays drawing of the model of open shell N−H···OC hydrogen-bonding in the OC(Y)−CHC(X)N−H six-membered cyclic system was chosen to study the open shell N−H···OC intramolecular hydrogen-bonding interactions37,38 and examine tunable group effects on the hydrogen bonding in this system. Another reason why we use the N−H···OC open shell system from Figure 1a as a study model is because the O C(Y)−CHC(X)N−H system, where X and Y H, F, Cl, Br, OH, NH2, CN, NO2, C(O)H, CCH, CH2CH, or CH3, has both a novel structure and unique linker that allows us to study and tune the group effects of the substituents on open shell N−H···OC interaction. In addition, this system has experimental relevance, as these compounds can be prepared or designed to study these group effect experimentally. When X and Y are electron-withdrawing or electrondonating groups, the hydrogen-bonding interaction of the

Figure 1. Open shell and closed shell model system are shown. The open shell is on the (a) and the closed shell is on the (b) pictures, respectively. (The simplest case is when X = Y = H. Oxygen, carbon, nitrogen, and hydrogen atoms are red, gray, blue and white, respectively.)

OC system has been connected by the normal CC−C group as a linker. To our knowledge, the effect of the substituents on this type of open-shell N−H···OC system has never been studied. This is a model system to systematically study effect that X and Y substituents, when introduced to each of the two C atoms forming this open shell N−H···OC sixmembered system, can have on the hydrogen bonding across the open-shell N−H···OC system.

■

COMPUTATIONAL METHODS Calculations were performed using the GAUSSIAN 09 suite of programs.29 Full geometry optimizations were performed using Schlegel’s method with tolerances of better than 0.001 Å for bond lengths and 0.01° for angles, and with a self-consistent field convergence of at least 10−9 of the density matrix. The residual root-mean-square (rms) forces were less than 10−4 a.u. Density functional theory calculations were performed using hybrid functional containing the unrestricted three parameter exchange functional of Becke and the LYP correlation functional (UB3LYP). Dunning’s augmented split valence correlation-consistent polarized valence double-ζ and triple-ζ quality basis sets (aug-cc-pVDZ and aug-cc-pVTZ) were used for calculations. The Dunning, aug-cc-pVDZ, basis set is used with the unrestricted second-order Møller−Plesset perturbation (UMP2) optimization calculations. Single point calculations B


Article

The Journal of Physical Chemistry A

open shell system is all longer than 1.022 Å, while the longest r(N···H) distance in the closed shell system is 1.016 Å as shown in Table 1. The OHN angle in in the open shell system stays about the same (ca. 133.9° ± 2°). Moreover, the closed shell H-bonding appears to have slightly shorter r(H···O) interaction distance than its open shell counterpart. In this work, the r(H···O) H-bond distance was examined to see if it is affected by inductive effects of the Y group, which should affect the open shell N−H bond distance. The UB3LYP/aug-cc-pVTZ results for these 12 substituents when X = H are listed in Table 2. From these data, one can observe that these N−H···OC interactions are present for all 12 functional groups. It should be noted that the distances between H and O atoms in the N−H···OC open shell system are all significantly smaller than 2.77 Å, which is the sum of van der Waals radii of H and O atoms. In Table 2, the values of trans N−H bond distances are almost the same (ca. 1.020 Å), when Y groups are varied. In contrast, the cis conformation exhibits an evolution of the N−H bond distance as Y is varied. When Y= EDG, the O···H bond distance is strengthened, so the r(O···H) distance is shortened, with corresponding lengthening of the N−H bond distance. In fact, when Y= NH2 and C2H3, the r(H···O) distances are the shortest and second shortest at 2.089 and 2.102 Å, respectively. As for the CO double bond, the same trend is also observed. For example, when Y = NO2, the r(O···H) distance is elongated to the longest distance of Figure 2b are the plots of N−H bond distance, rcis(N−H), and CO bond distance, rcis(CO), versus 2.202 Å, the N−H bond distance is shortest, 1.022 Å, and the CO double bond becomes the fourth shortest at 1.194 Å. Interestingly, the plot of N−H bond distance of the cisoid isomers versus r(O···H) looks logarithmic as shown in Figure 2a. Accordingly, the plot of CO bond distance of the cisoid isomers also shows a similar trend to the r(O···H) distance shown in Figure 2b. Thus, from the rcis(N−H) bond distance, a general trend is found; the rcis(N−H) varies inversely to the r(H···O) distance in the series of 12 substituents. The rcis(CO) distance shows a similar trend to the r(H···O) distance. Figure 2a and Figure 2b are the plots of N−H bond distance, rcis(N−H), and CO bond distance, rcis(CO), versus r(H···O) distance, respectively.

N−H···OC open shell systematically changes in structure and binding energy with the substituents. The 12 substituents that are used in this study as X and Y groups in Figure 1 can be divided into two groups: (1) the electron-donating group, EDG; (2) the electron-withdrawing group, EWG. We also sought to explore the difference in N−H···OC Hbonding between the open shell OC(Y)−CHC(X)N−H and its closed shell analogue OC(Y)−CHC(X)N−H2. The right image (b) of Figure 1 shows the closed shell system of OC(Y)−CHC(X)N−H2 to have six electrons in its π system, which include two double bonds and two partial double bonds in the six-membered structure. The open shell structure of in Figure 1a has another unique features when compared to its closed shell analogue in Figure 1b. First, the unpaired electron from nitrogen is delocalized between C3 and N5 atoms. Second, the dominant factors governing the open shell N−H···OC hydrogen bonding system39 arise from the electronic influence of different substituents. As shown in Table 1, in the open shell system, the N−H bond length is longer and Table 1. Comparison of Structural Features between the Open-Shell and Closed-Shell Model System with X = H r(N−H) bond length (Å)a

θ(O1H6N5) (deg)a

r(H···O) (Å)

entry

Y

open

closed

open

closed

open

closed

1 2 3 4 5 6 7 8 9 10 11 12

NH2 C2H3 C2H CH3 OH H C(O)H Br Cl F CN NO2

1.024 1.023 1.022 1.023 1.023 1.022 1.022 1.022 1.022 1.022 1.022 1.022

1.013 1.016 1.014 1.014 1.011 1.014 1.014 1.010 1.010 1.011 1.014 1.011

135.8 135.0 134.4 134.7 134.6 134.4 134.2 132.5 132.9 133.4 133.5 132.7

127.8 129.0 128.1 128.1 126.1 128.5 128.4 124.5 124.8 125.3 124.6 125.5

2.089 2.102 2.128 2.112 2.144 2.157 2.158 2.184 2.177 2.193 2.161 2.202

1.967 1.917 1.949 1.943 2.021 1.959 1.958 2.062 2.055 2.060 1.972 2.041

a

Bond length in Å, bond angle in degree.

the OHN angle greater than these two values from closed shell system, respectively. For example, the r(N···H) distance in the

Table 2. N−H and CO Bond Length (Å) Data of N−H···OC System with Y = 12 Substituents and X = H calculated at the UB3LYP/aug-cc-pVTZ level of theory N−H bond length (Å)a

CO bond length (Å)a

distance (Å)a

entry

Y

rtrans(N−H)b

rcis(N−H)

Δr(N−H)c

rtrans(CO)

rcis(CO)

Δr(CO)d

r(H···O)

1 2 3 4 5 6 7 8 9 10 11 12


1.020 1.020 1.020 1.020 1.020 1.020 1.020 1.020 1.020 1.020 1.020 1.020

1.024 1.023 1.022 1.023 1.023 1.022 1.022 1.022 1.022 1.022 1.021 1.021

−0.004 −0.003 −0.002 −0.003 −0.003 −0.002 −0.002 −0.002 −0.002 −0.002 −0.001 −0.001

1.217 1.223 1.220 1.217 1.205 1.216 1.222 1.181 1.184 1.185 1.215 1.186

1.226 1.231 1.229 1.226 1.214 1.223 1.229 1.192 1.194 1.192 1.223 1.194

−0.009 −0.009 −0.009 −0.009 −0.009 −0.007 −0.007 −0.011 −0.010 −0.007 −0.008 −0.008

2.089 2.102 2.128 2.112 2.144 2.157 2.158 2.184 2.177 2.193 2.161 2.202

a Bond length in Å. br(N···H) distances in closed shell system are all 1.020 Å. cΔr(N−H) = rtrans(N−H) − rcis(N−H). dΔr(CO) = rtrans(CO) − rcis(CO).

C


Article


Figure 2. (a) Plot of rcis(N−H) versus r(H···O) distance. (b) Plot of rcis(CO) versus r(H···O) distance. For an open shell, cyclic model system of OC(Y)−CHC(X)N−H when X = H, Y group is varied.

Table 3. N−H and CO Bond Length (Å) Data of N−H···OC System with X = 12 Substituents and Y = H Calculated at the UB3LYP/aug-cc-pVTZ Level of Theory N−H bond length (Å)a

a

CO bond length (Å)a

distance (Å)a

entry

X

rtrans(N−H)

rcis(N−H)

Δr(N−H)

rtrans(CO)

rcis(CO)

Δr(CO)

r(H···O)

1 2 3 4 5 6 7 8 9 10 11 12


1.021 1.021 1.020 1.021 1.020 1.020 1.024 1.016 1.017 1.019 1.020 1.024

1.018 1.021 1.022 1.021 1.019 1.022 1.022 1.025 1.024 1.021 1.023 1.023

0.003 0 −0.002 0 0.001 −0.002 0.002 −0.009 −0.007 −0.002 −0.003 0.001

1.219 1.216 1.216 1.217 1.219 1.216 1.216 1.216 1.216 1.217 1.214 1.214

1.225 1.224 1.224 1.224 1.225 1.223 1.224 1.223 1.223 1.224 1.222 1.222

−0.006 −0.008 −0.008 −0.007 −0.006 −0.007 −0.008 −0.007 −0.007 −0.007 −0.008 −0.008

2.111 2.086 2.096 2.109 2.135 2.157 2.138 2.094 2.096 2.147 2.102 2.110

Bond length in Å.

Figure 3. Plot of N−H···O hydrogen binding energy, Binding Energy, versus r(H···O), when X = H, Y group is varied. (a) UB3LYP/aug-cc-pVTZ plotted (in red), and M06-2X/aug-cc-pVTZ (in green) level of theory results are plotted together; (b) UMP2/aug-cc-pVTZ//UB3LYP/aug-ccpVTZ (in blue) and UCCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ (in yellow) level of theory results are plotted together.

in Y, the rtrans(N−H) distance irregularly varies, and shows no noticeable trend as the electronic properties of X are varied. For example, the r(H···O) is longest at 2.157 Å when X = Y = H, and r(H···O) is shorter than 2.157 Å no matter what X group is (EDG or EWG) while Y = H. The important feature noticed

Given the results for varying the Y substituent group, a study of the effects of varying the X substituent group while holding the Y group fixed (i.e., when Y = H) was also conducted. Data for the N−H, and CO bond lengths and r(H···O) distances are shown in Table 3. In contrast to its behavior under changes D


Article


Furthermore, we find that the (H6N5C4) angle is close to 109.9°, [although there is the slight variation in these angles (±1.0° from 110.0°); see the second column from the right in Table 4]. The trend, which states that the electron donating Y group strengthens the N−H···OC hydrogen-bond and the electron withdrawing agent weakens it, observed in Figure 4a, is also seen in Figure 4b. These findings are reproduced with the M06-2X/aug-cc-pVTZ data set. These results suggest that the tuning of the r(H···O) interaction is more a resonance cooperative effect across the six-membered ring of the OC(Y)−CHC(X)N−H system as shown in Figure 1; as one might expect this tuning influences both the (O1C2C3) and (H6N5C4) angles. This behavior supports the generality of these trends. One prominent characteristic of the hydrogen-bond is the relationship between the hydrogen-bond angle and the hydrogen-bond strength.37,38 For the intermolecular hydrogen-bond, from which many of these generalizations are deduced, the donor-proton-acceptor angle should be close to linear. This feature has been experimentally validated from structural studies from the Cambridge Structural Database.40 However, for intramolecular hydrogen-bonds, the donor− proton−acceptor angle is fixed by the molecular geometry of the system. As a consequence, donor−proton−acceptor angles should not be close to 180°. For the intramolecular hydrogenbond,37,38 the angle for the donor−proton−acceptor should be close to 158°.37,38 These angle characteristics for intermolecular and intramolecular hydrogen-bonds are based on structural studies from closed-shell systems. Are these generalizations valid for open-shell systems? The N−H···OC open-shell intramolecular hydrogen bond could provide some insights into this question. A plot of the N−H··· OC angle with binding energy in Figure 5 (the hydrogen bond energy is calculated at the UMP2/aug-cc-pVTZ// UB3LYP/aug-cc-pVTZ level of theory), shows that the angle ranges from 132.7° to 135.8°, and are linearly dependent on the bond energy. In fact, the data in Figure 5 show that the strongest intramolecular N−H···OC open shell hydrogenbond (7.1 kcal/mol) is when Y = C2H3, which has second largest angle of 135.0°, and the weakest open shell hydrogenbond is when Y = NO2, which has the smallest binding energy and the smallest angle of 3.3 kcal/mol and 132.7°, respectively. We have found for the N−H···OC open-shell intramolecular hydrogen-bond that there is a linear relation between hydrogen-bond angle and N−H···OC hydrogen-bond energy. An examination of the N−H stretching frequency, using IR or Raman spectrosocpic techniques, is a cruical experimental proble for the N−H···OC hydrogen bonding interaction. The connection between intramolecular hydrogen bonding and changes in infrared stretching motions have been noted by Kuhn.41−43 These studies have led to the general assertion that the IR X-H stretching frequency red-shifts relative to its non Hbonded analogue. However, many of these findings are based on results for closed-shell molecular systems. To examine if this is vaild for the open-shell N−H···OC model system in this study, in Table 5 the UB3LYP/aug-cc-pVTZ results for the N− H stretching frequency is presented for all 12 functional groups. When Y = NO2 (very strong EWG), the N−H frequency is 3434 cm−1, which is the largest N−H vibration among the series. The smallest N−H vibrational frequency is for Y = NH2, which has a frequency of 3404 cm−1, suggesting that in the series the strongest N−H···OC interaction occurs for Y =

for the open shell N−H···OC model system, from Tables 2 and 3 is that the group substituent effects are mainly observed for the case when (X = H, Y = varied), but not for the case when (X = varied, Y = H). These data suggest that the main tunability of the r(H···O) interaction occurs from the electronic effects of the Y group on the carbonyl CO bond.39 The E2 interaction energy from the natural bond orbital analysis which was performed at the UB3LYP/aug-cc-pVDZ level of theory allows for the N−H···OC interaction to be isolated from other molecular structural changes and can give more direct insight into the stabilizing interaction from the group substitution. The plot of E2 versus the r(H···O) shows a linear trendi.e., the greater the overlap, the larger the E2 and the shorter the H and O distance (shown in Figure S7 in the Supporting Information). This suggests that the distance between the hydrogen and oxygen atoms correlates linearly with the degree of overlap in the N−H···OC open shell system. Similar to Figure S7 (in Supporting Information), Figure 3 shows that there is a linear correlation of the r(H···O) and the N−H···OC hydrogen bond energy. A short r(H···O) distance corresponds with a large N−H···OC hydrogen bond energy. When the substituents are EDG, such as NH2, CH3, C2H3, C2H, OH, the r(H···O) distances are shorter, based on UB3LYP/aug-cc-pVTZ level of theory, ranging from 2.089 to 2.144 Å. In contrast, when the substituents are strong EWG like F or NO2, the r(H···O) distances are longer, corresponding to 2.193 and 2.202 Å, respectively. We have further explored how much the bond angle changes affect r(H···O) interaction distances. The two key angles are the θ(O1C2C3) and θ(H6N5C4) angles whose labeling is shown Figure 1. The two angles, θ(O1C2C3) and θ(H6N5C4) involved in the N−H···OC interactions have sp2 and sp3 hybridized carbon centers, respectively. However, the θ(O1C2C3) angle, which has the carbon center with the Y group, does show variance with the r(H···O) distance as shown in Table 4. Figure Table 4. Two Bond Angles of θ(O1C2C3) and θ(H6N5C4) from the Open Shell N−H···OC System with Y = 12 Substituents and X = H Calculated at the UB3LYP/aug-ccpVTZ Level of Theory Y

θ(O1C2C3) (deg)a

θ(H6N5C4) (deg)b

r(O···H) (Å)a


123.7 122.1 123.7 122.2 126.3 124.9 125.6 128.2 127.9 129.2 125.6 130.4

109.2 109.5 109.9 109.5 109.9 109.9 110.2 110.8 110.6 110.5 110.6 111.0

2.089 2.102 2.128 2.112 2.144 2.157 2.158 2.184 2.177 2.193 2.161 2.202

a

Bond angles in degree; bond lengths in Å. bThere is the slight variation in these θ(H6N5C4) angles (±1.0° from 110.0°).

4a shows that this trend is linear. When Y = NO2 or F (strong EWG), this (O1C2C3) angle is close to 130°. However, when Y is the strong EDG (e.g., C2H3), this (O1C2C3) angle is only 122°. The group effect from the Y group is significant because there is an 8 deg difference when changing the Y group from a strong EWG, e.g., NO2, to a strong EDG, e.g., C2H3. E


Article


Figure 4. Plot of (a) angle (O1C2C3) versus r(H···O) (Å); (b) angle (H6N5C4) versus r(H···O) (Å), when X = H, Y group is varied. Red colors are UB3LYP/aug-cc-pVTZ data and green are M06-2X/aug-cc-pVTZ data.

changes in the N−H bond resulting from the hydrogen bonding interaction show a highly correlated association between the effect of intramolecular hydrogen bonding on the structure of the N−H bond and its N−H vibration; the redshift scales linearly with the geometry changes in the N−H bond. Logansen44 noted a strong correlation between hydrogen bond energy and the intensity of the X-H stretching motion for closed shell systems. An examination of the results in Table 5 show that the intensity of the N−H stretching mode increases for all 12 functional groups. A plot of the N−H···OC hydrogen-bond interaction energy (Figure 6) with changes in intensity between cisoid and transoid conformers for the N−H stretch vibration, show a strong correlation. Results in Table 5 and Figure 6 suggest that the red-shifts in the N−H stretching frequency and increases in its intensity are useful probes for hydrogen bonding in the open-shell N−H···OC system. Moreover, these results represent the first validation of a criterion for hydrogen bonding in an open-shell system. Frey and Leutwyler,45 noted that the closed-shell amide N−H···O C hydrogen bonds are centrally involved in the structure and function of DNA and RNA. In fact, most enzymatic systems inside biological molecules are mainly dependent on functionality and sequence of amino acid residues.

Figure 5. Plot of angle (N5−H6···O1) versus binding energy, when X = H and the Y group is varied. Blue: UMP2/aug-cc-pVTZ//UB3LYP/ aug-cc-pVTZ level of theory. Yellow: UCCSD(T)/aug-cc-pVTZ// M06-2X/aug-cc-pVTZ.

NH2 and is consistent with the shortest r(H···O) distance of 2.089 Å. An examination of the N−H frequency between the cisoid and transoid conformations (Table 5) shows that for all the 12 functional groups the N−H stretching motion decreases in frequency, i.e. red-shifts relative to the non H-bonded transoid conformation. In fact, a plot of the red-shifts with the

Table 5. N−H Vibrational Frequency (cm−1) Data for the N−H···OC Interaction with 12 Substituents Calculated at the UB3LYP/aug-cc-pVTZ Level of Theory with X = H and Y = 12 Substituents N5−H6 vibrational frequencya

a

N5−H6 vibrational intensitya

entry

Y

vcis(N−H)

vtrans(N−H)

Δv(N−H)

Icis(N−H)

Itrans(N−H)

ΔI(N−H)

1 2 3 4 5 6 7 8 9 10 11 12


3404 3413 3423 3417 3418 3422 3426 3428 3432 3426 3432 3434

3444 3444 3446 3446 3448 3445 3447 3456 3453 3452 3448 3453

−40 −31 −23 −29 −30 −23 −21 −28 −21 −26 −16 −19

32.3 22.1 19.9 22.4 20.8 18.4 14.7 11.9 13.3 15.8 16.8 11.6

1.3 2.0 3.1 1.8 2.3 2.4 3.9 6.6 5.4 4.2 6.1 7.4

31.0 20.1 16.8 20.6 18.5 16.0 10.8 5.3 7.9 11.6 10.7 4.2

Frequency in cm−1. F


Article


Figure 6. (a) Plot of Δv(N−H)(cis‑tran) vs the Δr(N−H)(trans−cis). (b) Plot of the N−H···OC binding energy vs ΔI(N−H)

(cis−trans).

bonding in an open-shell system, namely that concurrent redshifts in the N−H stretching frequency and increases in its intensity are useful probes for hydrogen bonding in the openshell N−H···OC system. In particular, the differences encountered between equivalent open and closed shell systems are unusually long r(N···H) distances, >1.022 Å, in the open shell system as opposed to about 1.01 Å in closed shell system. . The group effect on N−H···OC interaction in the open shell can also be better studied by monitoring θ(O1C2C3) than that in the closed shell system. The observed trends and findings have been validated using three different methods (UB3LYP, M06-2X, and UMP2) with two different basis sets.

Furthermore, the weak H-bonding interactions inside these systems are very important in enzyme-catalyzed reactions. Scheiner46 noted that β-helix and β sheets represent two of the most common classes of protein secondary structure. It is commonly believed that the stability of these protein structures are in large measure due to the strengths of the hydrogen bonds that link the NH donor of one peptide group with the carbonyl O of another.47 Proteins are major targets for free radical attack due to their high concentrations in biological systems.48 Free radical attack can result in protein fragmentation and denaturation, which in turn alters the hydrogen bonding patterns leading to protein unfolding and possible formation of a new tertiary structure.48−50 Consequently, an accurate description of the N−H···OC hydrogen bond geometries and interaction energies are important for understanding biological functionality after radical attack in proteins. Findings from this study suggest that the control of open-shell N−H··· OC interactions can be tuned by substituents at carbonyl sites. Changing the substituents at the carbonyl group is more effective than changing the groups at the carbon carrying the amino groups. The electronic properties of the Y group are found to exert a significant effect on the open-shell N−H···O C hydrogen bonding.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b00144. Supplementary files SF1, including the data from UB3LYP/aug-cc-pVDZ calculations to affirm the trends, SF2, containing all the UB3LYP/aug-cc-pVTZ optimized geometry, SF3, containing all the Mulliken spin density for UB3LYP/aug-cc-pVTZ from the optimized structure, SF4, containing the Mulliken charges for UB3LYP/augcc-pVTZ from the optimized structures, and SF5, containing ZPE data for optimized geometries at the UB3LYP/aug-cc-pVTZ level of theory (PDF)

■

CONCLUSIONS By studying the group effects on the N−H···OC interaction, it was found that there is a structural effect of functional group substitution on the open shell N−H···OC interaction. There is a general trend within the 12 substituents studied. They can be divided into two main groups as the electron donating group, EDG (NH2, CH3, CHCH2, CCH and OH), and the electron withdrawing group, EWG (C(O)H, F, Cl, Br, CN, NO2). It was found that the extent of electron-donating ability of the Y group can increase the hydrogen bonding interaction in the N−H···OC system. In the present work, we have shown for the first time that there is a group effect on the open shell N−H···OC interaction when the Y group is varied while X = H, suggesting that the hydrogen bonding interaction from the OC(Y)−CHC(X)N−H system can be molecularly tuned. Our findings show that there is a linear relation between hydrogen-bond angle and N−H···OC hydrogenbond energy for this open-shell system. Moreover, our findings represent the first validation of a criterion for hydrogen

■

AUTHOR INFORMATION

Corresponding Authors

*(N.L.) E-mail: [email protected]. *(J.S.F.) E-mail: [email protected]. *(E.N.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Authors gratefully acknowledge the supports from Purdue University. N.L., who spent sabbatical leave at Purdue University, thanks the Ministry of Science and Technology of Taiwan for financial support. G


Article


■

(24) Misochko, E. Y.; Benderskii, V. A.; Goldschleger, A. U.; Akimov, A. V.; Shestakov, A. F. Formation of the CH3-HF Complex in Reaction of Thermal F Atoms with CH4 in Solid Ar. J. Am. Chem. Soc. 1995, 117, 11997−11998. (25) Francisco, J. S.; Sander, S. P. Existence of a Chlorine Oxide and Water (ClO-H2O) Radical Complex. J. Am. Chem. Soc. 1995, 117, 9917−9918. (26) Leopold, K. R.; Canagaratna, M.; Phillips, J. A. Partially Bonded Molecules from the Solid State to the Stratosphere. Acc. Chem. Res. 1997, 30, 57−64. (27) Chen, Y. F.; Dannenberg, J. J. Cooperative 4-Pyridone H-Bonds with Extraordinary Stability. A DFT Molecular Orbital Study. J. Am. Chem. Soc. 2006, 128, 8100−8101. (28) Gregoret, L. M.; Rader, S. D.; Fletterick, R. J.; Cohen, F. E. Hydrogen Bonds Involving Sulfur Atoms in Proteins. Proteins: Struct., Funct., Genet. 1991, 9, 99−107. (29) Frisch, J. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, Revision B.01; Gaussian, Inc.: Wallingford, CT, 2010. (30) Glendening, E. D.; Landis, C. R.; Weinhold, F. NBO 6.0: Natural Bond Orbital Analysis Program. J. Comput. Chem. 2013, 34, 1429−1437. (31) Zhao, X.; Wang, X.-Z.; Jiang, X.-K.; Chen, Y.-Q.; Li, Z.-T.; Chen, G.-J. Hydrazide-Based Quadruply Hydrogen-Bonded Heterodimers. Structure, Assembling Selectivity, and Supramolecular Substitution. J. Am. Chem. Soc. 2003, 125, 15128−15139. (32) Tatamitani, Y.; Liu, B.; Shimada, J.; Ogata, T.; Ottaviani, P.; Maris, A.; Caminati, W.; Alonso, J. L. Weak, Improper, C−O···H−C Hydrogen Bonds in the Dimethyl Ether Dimer. J. Am. Chem. Soc. 2002, 124, 2739−2743. (33) López, J. C.; Caminati, W.; Alonso, J. L. The C−H···π Hydrogen Bond in the Benzene−Trifluoromethane Adduct: A Rotational Study. Angew. Chem., Int. Ed. 2006, 45, 290−293. (34) Lu, N.; Ley, R.; Cotton, C.; Chung, W.-C.; Francisco, J. S.; Negishi, E.-I. Molecular Tuning of the Closed Shell C−H···F−C Hydrogen Bond. J. Phys. Chem. A 2013, 117 (34), 8256−8262. (35) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Intermolecular Interactions from a Natural Bond Orbital, Donor-Acceptor Viewpoint. Chem. Rev. 1988, 88, 899−926. (36) Vargas, R.; Garza, J.; Hay, B. P.; Dixon, D. A. How Strong is the Cα−H···OC Hydrogen Bond? J. Am. Chem. Soc. 2000, 122, 4750− 4755. (37) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; et al. Definition of the Hydrogen Bond (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83, 1637−1641. (38) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; et al. Defining the Hydrogen Bond: An Account (IUPAC Technical Report). Pure Appl. Chem. 2011, 83, 1619−1636. (39) Vargas, R.; Garza, J.; Friesner, R. A.; Stern, H.; Hay, B. P.; Dixon, D. A. Strength of the NH···OC and CH···OC Bonds in Formamide and N-Methylacetamide Dimers. J. Phys. Chem. A 2001, 105, 4963−4968. (40) Allen, F. H. The Cambridge Structural Database: A Quarter of A Million Crystal Structures and Rising. Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58 (3), 380−388. (41) Kuhn, L. P. The Hydrogen Bond. I. Intra- and Intermolecular Hydrogen Bonds in Alcohols. J. Am. Chem. Soc. 1952, 74, 2492−2499. (42) Kuhn, L. P. The Hydrogen Bond. II. The Intramolecular Bond in Cyclic 1,2-Diols. J. Am. Chem. Soc. 1954, 76, 4323−4326. (43) Kuhn, L. P. The Hydrogen Bond. III. The Effect of the Size of Substituents upon the Length of the Hydrogen Bond in Various Substituted 1,2-Diols. J. Am. Chem. Soc. 1958, 80, 5950−5954. (44) Iogansen, A. V. Direct Proportionality on the Hydrogen Bonding Energy and the Intensification of the Stretching (XH) Vibration in Infrared Spectra. Spectrochim. Acta, Part A 1999, 55, 1585−1612.

REFERENCES

(1) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, J.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P. Defining the Hydrogen Bond: An Account (IUPAC Technical Report). Pure Appl. Chem. 2011, 83, 1619−1636. (2) Pauling, L.; The Nature of the Chemical Bond; Cornell University Press: Ithaca, NY, 1947; First ed. was published in 1939. (3) Desiraju, G. R.; Steiner, T.; The Weak Hydrogen Bonding in Structural Chemistry and Biology; Oxford University Press: New York, 1999. (4) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman: San Francisco, CA, 1960. (5) Jeffrey, J. A. Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. (6) Karty, J. M.; Wu, Y.; Brauman, J. I. The RS−·HSR Hydrogen Bond: Acidities of α,ω-dithiols and Electron Affinities of Their Monoradicals. J. Am. Chem. Soc. 2001, 123, 9800−9805. (7) Cleland, W. W.; Frey, P. A.; Gerlt, J. A. The Low Barrier Hydrogen Bond in Enzymatic Catalysis. J. Biol. Chem. 1998, 273, 25529−25532. (8) Fonseca-Guerra, C.; Bickelhaupt, F. M.; Snijders, J. G.; Baerends, E. J. The Nature of the Hydrogen Bond in DNA Base Pairs: The Role of Charge Transfer and Resonance Assistance. Chem. - Eur. J. 1999, 5, 3581−3594. (9) Scheiner, S. J. Relationship between Strength of Hydrogen Bond and Barrier to Proton Transfer. J. Mol. Struct.: THEOCHEM 1994, 307, 65−71. (10) Scheiner, S.; Redfern, P. Quantum Mechanical Test of Marcus Theory. Effects of Alkylation Upon Proton Transfer. J. Phys. Chem. 1986, 90, 2969−2974. (11) Leuchs, M.; Zundel, G. Easily Polarizable Hydrogen Bonds and Solvate Structure in Aqueous Solutions of Acids with pKa < 1. Can. J. Chem. 1980, 58, 311−322. (12) Suma, K.; Sumiyoshi, Y.; Endo, Y. The Rotational Spectrum of the Water-Hydroperoxy Radical (H2O-HO2) Complex. Science 2006, 311, 1278−1281. (13) Frisch, M. J.; Del Bene, J. E.; Binkley, J. S.; Schaefer, H. F. Extensive Theoretical Studies of the Hydrogen-bonded Complexes (H2O)2, (H2O)2H+, (HF)2, (HF)2H+, F2H−, and (NH3)2. J. Chem. Phys. 1986, 84, 2279−2289. (14) Miller, C. E.; Francisco, J. S. The Formation of a Surprisingly Stable HO2-H2SO4 Complex. J. Am. Chem. Soc. 2001, 123, 10387− 10388. (15) Aloisio, S.; Francisco, J. S Structure and Energetics of Hydrogen Bonded HOx-HNO3 Complexes. J. Phys. Chem. A 1999, 103, 6049− 6053. (16) Francisco, J. S. Deprotonation Energy of HO2: Basis Set Limit Energies. J. Chem. Phys. 2001, 115, 6373−6375. (17) Janaway, G. A.; Zhong, M.; Gatev, G. G.; Chabinyc, M. L.; Brauman, J. I. [FHNO]−: An Intermediate in a Spin-Forbidden Proton Transfer Reaction. J. Am. Chem. Soc. 1997, 119, 11697−11698. (18) Mihalick, J. E.; Gatev, G. G.; Brauman, J. I. Electron Photodetachment Spectroscopy of Solvated Anions: RO·HF− or ROH·F−? J. Am. Chem. Soc. 1996, 118, 12424−12431. (19) Alkorta, I.; Rozas, I.; Elguero, J. Radicals as hydrogen bond acceptors. Ber. Bunsen. Phys. Chem. 1998, 102, 429−435. (20) Hammerum, S. Alkyl Radicals as Hydrogen Bond Acceptors: Computational Evidence. J. Am. Chem. Soc. 2009, 131, 8627−8635. (21) Alapati, S. V.; Johnson, J. K.; Sholl, D. S. Large-Scale Screening of Metal Hydride Mixtures for High-Capacity Hydrogen Storage from First-Principles Calculations. J. Phys. Chem. A 2008, 112, 5258−5262. (22) Wang, B.-Q.; Li, Z.-R.; Wu, D.; Hao, X.-Y.; Li, R.-J.; Sun, C.-C. Single-electron hydrogen bonds in the methyl radical complexes H3C···HF and H3C···HCCH: an ab initio study. Chem. Phys. Lett. 2003, 375, 91−95. (23) Sander, W.; Roy, S.; Polyak, I.; Ramirez-Anguita, J. M.; SanchezGarcia, E. The Phenoxyl Radical−Water Complex-A Matrix Isolation and Computational Study. J. Am. Chem. Soc. 2012, 134, 8222−8230. H


Article

The Journal of Physical Chemistry A (45) Frey, J. A.; Leutwyler, S. An Ab initio Benchmark Study of Hydrogen Bonded Formamide Dimmers. J. Phys. Chem. A 2006, 110, 12512−12518. (46) Scheiner, S. Contributions of NH··O and CH··O H-Bonds to the Stability of β-Sheets in Proteins. J. Phys. Chem. B 2006, 110, 18670−18679. (47) Pauling, L.; Corey, R. B. Configurations of Polypeptide Chains with Favored Orientations Around Single Bonds. Proc. Natl. Acad. Sci. U. S. A. 1951, 37, 729−740. (48) Davies, M. J. The Oxidative Environment and Protein Damage. Biochim. Biophys. Acta, Proteins Proteomics 2005, 1703, 93−109. (49) Davies, K. J. A. Protein Damage and Degradation by Oxygen Radicals. I. general aspects. J. Biol. Chem. 1987, 262, 9895−9901. (50) Anfinsen, C. B. Principles that Govern the Folding of Protein Chains. Science 1973, 181, 223−230.

I


Molecularly Tuning the Radicaloid NâHÂ·Â·Â·O C Hydrogen Bond - The

Recommend Documents

Molecularly Tuning the Radicaloid NâHÂ·Â·Â·O C Hydrogen Bond - The