Article pubs.acs.org/JPCA
Microwave Spectrum for a Second Higher Energy Conformer of Cyclopropanecarboxylic Acid and Determination of the Gas Phase Structure of the Ground State Aaron M. Pejlovas,† Wei Lin,‡ and Stephen G. Kukolich*,† †
Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States Department of Chemistry, University of Texas Rio Grande Valley, Brownsville, Texas 78520, United States
‡
ABSTRACT: Microwave spectra for a higher-energy conformer of cyclopropanecarboxylic acid (CPCA) were measured using a Flygare−Balle-type pulsed-beam Fourier transform microwave spectrometer. The rotational constants (in megahertz) and centrifugal distortion constants (in kilohertz) for this higher-energy conformer are A = 7452.3132(57), B = 2789.8602(43), C = 2415.0725(40), DJ = 0.29(53), and DJK = 2.5(12). Differences between rotational constants for this excited-state conformation and the ground state are primarily due to the acidic OH bond moving from a position cis relative to the cyclopropyl group about the C1−C9 bond to the more stable trans conformation. Calculations indicate that the relative abundance of the higher-energy state should be 15% to 17% at room temperature, but the observed relative abundance for the supersonic expansion conditions is about 1%. The measurements of rotational transitions for the trans form of CPCA were extended to include all of the unique 13C singly substituted positions. These measurements, along with previously measured transitions of the parent and −OD isotopologues, were used to determine a best-fit gas-phase structure.
1. INTRODUCTION Cyclopropane and its derivatives are reactive, versatile compounds that can be found throughout nature.1 They are important throughout biology2 and can be synthesized and utilized for many important reactions.3,4 Structural information on these types of molecules is important to obtain to understand the chemistry and reactivity behind these enzymatic processes in biology or reactions performed in the lab. Infrared, Raman, microwave, and X-ray crystal structure studies have been reported previously for cyclopropanecarboxylic acid (CPCA).5−8 To obtain a more accurate gas-phase structure, transitions from 13C singly substituted isotopologues were needed to continue from the previous work. This work extends the microwave spectroscopic work to include these 13C measurements and allows us to obtain a best-fit gas-phase structure of CPCA. The Gaussian program with density functional theory and MP2 methods have been reasonably well tested and successful for ground-state singlet molecules but should really be tested further for excited states. This work provides an example test. Many molecules have multiple low-energy conformations, adopted through rotation about single bonds or a change in © XXXX American Chemical Society
local atomic geometry. These multiple conformations often have important effects on initiating reactions and on reaction pathways. The effects are more important for catalyzed reactions as catalysis is often very sensitive to conformation and the structure of substrates. Conformational behavior of biologically active molecules is a critical feature of many of their reactions. Often higher-energy conformers can be more reactive than the ground states or may negatively impact reactions. Protein conformational heterogeneity and dynamics are known to play an important role in enzyme catalysis.9,10 Amino acids, such as serine, exhibit multiple low-energy conformations, and these were characterized and measured by Alonso et al.11 using microwave spectroscopy. Other examples of conformational studies important in biochemistry are nicotinic acid12 and α-Dglactose.13 Calculations involving potential energy scans for the different conformers of CPCA have been done previously.14 The results predicted that the two lowest-energy conformers are separated Received: July 13, 2015 Revised: September 10, 2015
A
DOI: 10.1021/acs.jpca.5b06733 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
after ∼2 h of pulsing only the CPCA sample through the pulsed-valve at ∼60 °C. The measured transitions for this new high-energy conformer are shown in Table 1.
by only a few hundred wavenumbers. These conformers are shown in Figure 1a,b for the higher-energy and low-energy
Table 1. Quantum Number Assignments and Frequencies for the Higher-Energy Conformer of CPCA
a
J′
Ka′
Kc′
J″
Ka″
Kc″
frequencies (MHz)
m−ca
1 1 2 2 3 4 1 2 2 2
1 0 1 0 1 1 1 1 0 0
0 1 1 2 2 3 1 2 2 2
1 0 2 1 3 4 0 1 1 1
0 0 0 1 0 0 0 1 0 1
1 0 2 1 3 4 0 1 1 0
5037.232 5204.936 5433.717 5725.706 6068.218 6984.654 9867.375 10 035.056 10 388.154 10 784.644
−4 4 6 −4 4 −5 −4 −2 −4 10
The measured−calculated (m−c) values are in kilohertz.
Measurements on the low-energy CPCA conformer, corresponding to the CPCA molecule shown in Figure 1b, were extended to include all of the single 13C-substituted positions, with all transitions being measured at natural abundance concentrations. These newly measured rotational transitions from these singly substituted isotopologues are shown in Table 2. The numbering scheme for the substituted atoms is also shown in Figure 1b. These new 13C measurements were also taken at room temperature, since a parent test signal was a very strong single shot after significant pulsing time at a higher temperature (60 °C).
Figure 1. Structures of the two conformers of CPCA. (a) The (cis) conformer is the higher-energy conformer. (b) The lower-energy (trans) conformer. The difference in energies of these two conformers is 373.1 cm−1, obtained from the B3LYP/aug-cc-pVQZ calculation.
3. CALCULATIONS Potential energy scans for rotation of the carboxylic acid moiety about the C1−C9 bond of the CPCA monomer were previously performed, resulting in calculated abundances of each conformer to be 85% and 15% at room temperature under equilibrium conditions.14 From this study, the barrier for the rotation about this C1−C9 bond was calculated to be 1400− 2100 cm−1. Additional optimization calculations using B3LYP/ aug-cc-pVQZ calculations of the two conformers were also performed using the Gaussian 09 suite17 at the University of Arizona. The energy difference between the conformers was calculated to be 373.1 cm−1, with the conformer in Figure 1a having higher energy. The calculated a- and b-dipole moment components of this higher-energy conformer were 2.02 and 1.13 D, respectively, so both a- and b-type transitions were expected to be observed. However, these transitions will be weaker due to this conformer having higher energy. The calculated values of the rotational constants from the B3LYP calculation and the previous theoretical study14 are compared to the experimentally fit values for this high-energy conformer in Table 3. Simple calculations to predict the rotational constants of the 13 C isotopologues were performed by calculating the ratio of the experimental versus calculated rotational constants of the parent to obtain a scale factor. This scale factor was multiplied by the calculated rotational constants of the isotopologues, which were found by changing the mass of one of the carbon atoms within the parent molecule and recalculating the moments of inertia. The rotational constants obtained by correcting the calculated values of the rotational constants with
conformers, respectively. The high-energy conformer has the acidic −OH bond cis to the cyclopropyl group, relative to the C1−C9 bond, and the low-energy conformer is in a trans configuration. The abundances predicted for equilibrium conditions at room temperature were 85% (low-energy conformer) and 15% (higher-energy conformer) from this computational study.14 We performed our own calculations (details in Section 3, below) and predicted that the difference in energies of these conformers was 321 cm−1, which agrees fairly well with the previous study. From these ab initio calculations, it is reasonable that rotational transitions from this high-energy conformer should be observed but significantly weaker than the previously measured low-energy conformer. In the work by Marstokk et al.,7 this higher-energy conformer was searched for in the 26.5−38.0 GHz range but was not observed, most likely due to the low abundance and low populations for the high-J transitions. In the present work, we report the microwave measurements and fits to determine rotational and centrifugal distortion constants of this higher-energy conformer of CPCA.
2. MICROWAVE MEASUREMENTS The CPCA sample (95%) was purchased from Sigma-Aldrich and was used without further purification. The sample was transferred to a glass sample cell and was attached to the pulsed-valve (General Valve series 9). Microwave measurements were taken using a Flygare−Balle-type, pulsed-beam Fourier transform microwave spectrometer that has been described previously.15,16 Measurements on the high-energy CPCA conformer corresponding to the geometry of the CPCA molecule shown in Figure 1a were taken at room temperature B
DOI: 10.1021/acs.jpca.5b06733 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Table 2. Quantum Number Assignments and Frequencies (Megahertz) for the 13C Isotopologues of the Cyclopropanecarboxylic Acid Low-Energy Trans Conformer
a
J′
Ka′
Kc′
J″
Ka″
Kc″
1 2 1 2 1 2 2
0 0 1 1 1 0 1
1 2 0 1 1 2 1
0 1 1 2 0 1 1
0 1 0 0 0 0 1
0 1 1 2 0 1 0
m−ca
13
C(3&6)
5027.498 5152.397 5212.357 5553.572 9914.347 100 039.255
2 −2 1 −1 −3 2
13
C(9)
5092.480 5259.474 5249.158 10 000.827 10 167.814
m−ca −4 2 0 1 0
13
C(1)
m−ca
5095.068
2
5203.388
4
9952.100 10 172.253 10 536.453
−4 4 −3
The measured−calculated (m−c) frequencies are in kilohertz.
distortion constants were held fixed to the values obtained by Marstokk et al.,7 due to these constants being measured for high J and K transitions and so likely to be more accurately determined. Also in Table 4 are the rotational constants from the best-fit structure (under the “calculated” column) and the deviation of these constants for the best-fit structure from the experimentally determined values (m−c). The calculated rotational constants for the best-fit structure and its isotopologues had a standard deviation of 0.31 MHz from the experimentally determined values. The fit for the rotational and centrifugal distortion constants of the higher-energy CPCA conformer were also performed using the Pickett SPFIT program and are shown in Table 5, along with our calculated B3LYP/aug-cc-pVQZ rotational constants and the rotational constants obtained from a previous theoretical study.14 The rotational constants calculated from the previous study of this conformer were within
