I would like to thank the professors of chemical engineering for their support during my research efforts in the chemistry department. Boron-nitrogen (B-N) and boron-phosphorus (B-P) dative bond-stretching vibrational frequencies were studied using vibrational spectroscopic methods and electronic structure calculations.
Dative Bonding and Applications
There has been great controversy over the physical properties of the boron-nitrogen dative bond over many years, with vibrational assignments of its stretching frequency ranging from cm-1. To continue the study of the bond stretching frequencies, boron-phosphorus (B-P) bonds are also investigated.
Spectroscopic Methods
- Vibrational Spectroscopy
- The Harmonic Oscillator
- Normal Modes
- Raman Spectroscopy
- Surface Enhanced Raman Spectroscopy
The stiffness of the spring is measured by the force constant k. The potential energy of this spring system is shown in equation 1.3.5. The induced dipole moment in a molecule is equal to the product of the polarizability and the applied electric field.
Computational Methods
Background
Ψ is the n-electron wave function, which depends on the identity and positions of the nuclei and the total number of electrons. Ĥ is the Hamiltonian operator, equation 1.12, which determines the kinetic and potential energy for each of the particles.4,5. 1.12).
Computational Methods
In this equation, ZA is the nuclear charge, MA is the mass of nucleus A, me is the mass of the electron, RAB is the distance between nuclei A and B, rij is the distance between electrons i and j, riA is the distance between electron i and nucleus A, ε0 is the permittivity of free space, ћ is Planck's constant divided by 2π, and is the Laplace operator of particle i. M06-2X is a fairly new method that has been proven to predict dative bonds well, and was therefore also used in this study for comparison.12 The INTDER2005 program13 was also used to determine the total energy distributions (TEDs) that represent the normal characterize modes of each bond. dative bound system.
For the dative bond stretching region, B3LYP tends to underestimate the vibrational energy while MP2 tends to overestimate. -2X is a fairly new method that has proven to predict well the data connections, so it was also used in this study for comparison.12 The program INTDER200513 was also used to determine the total energy distributions (TEDs) characterizing the normal modes of each dative. connected system.
Introduction
This new molecular class serves as the first general solution where stationary stability and the ability to slowly release boric acids in situ are incorporated.1-3 The key to the success of these molecules as synthetic reagents is the dative B-N bond. Due to the stability of MIDA esters, they are also ideal candidates for the spectroscopic study of B-N dative bonds. Here, we compare experimental infrared, Raman, and surface-enhanced Raman spectroscopy (SERS) spectra of MIDA methylboronic acid (MBA) ester (the structure is shown in Figure 2.1) with theoretical predictions to describe the physical properties of the dative B-N bond contained in this molecule. .
At the MP2 level, which predicted the B-N stretching between 678 and 690 cm-1, the use of larger basis sets with more diffuse features also did not improve the agreement between experiment and theory in terms of the location of the B-N stretching frequency. Calculations performed using the QCISD method lowered the energy of the B-N stretching motion by 13 cm-1 compared to MP2, but the authors recommended the use of the CCSD(T) coupled cluster method with large basis sets, which they could not perform at time due to disk space limitations. As a result, B3LYP cannot be expected to provide reasonable vibrational frequencies of the title compound in the B-N stretching region.
We compare our experimental spectra with the results of ab initio calculations using the MP2 and M06-2X methods and different sized basis sets to determine the location of the B-N stretching frequency of the dative bond in the MBA MIDA ester.
Experimental and Theoretical Methods
Spectroscopic Methods
Theoretical Methods
Results
Spectroscopic Results
The resulting spectrum for the 514.5 nm excitation (shown in Figure 2.2) represents the sum of an average of 20 individual scans, exhibits a relatively flat baseline, and agrees well with the spectra obtained using the Kr ion 676.4 nm laser line. Interestingly, spectra obtained by the Kr ion 647.1 nm laser line (not shown) also showed a large polynomial background. It is observed that the positions of the peaks in the Raman and infrared experimental spectra closely match each other.
However, at the lower energy end of the spectrum, the vibrational modes are relatively more pronounced in Raman spectra than in the infrared spectrum. This is consistent with previous observations by Odom and colleagues in the case. The carbonyl stretches at 1700 cm-1 correlate well with each other in both Raman and infrared spectra and appear as a doublet, as does the location of the C-H stretches.
There are also additional peaks evident in the Raman spectrum using 514.5 nm excitation that are not observed when 676.4 nm is used.
SERS Results
The prominence of these features suggests a close interaction between MBA MIDA ester and the silver substrate. Compared to theoretical predictions, most peaks in this region of the vibrational spectrum correlate with the motion of the nitrogen atom and its three adjacent CHn groups. The intensity of this peak may indicate an interaction between the nitrogen atom and silver atoms in the substrate in some of the MBA MIDA ester molecules69-71.
Theoretical Results
Discussion
Figures 2.5 and 2.6 show a detailed comparison between experiment and theory in the region associated with B-N stretching. To further put this into perspective, at the MP2/6-311G(2df,2pd) level of theory, only 7 of the 60 normal modes have a major contribution from a single simple motion. All experimentally observed peaks in Figures 2.5 and 2.6 are accounted for by both levels of theory.
Both MP2 and M06-2X predict four peaks in this spectral window, which explains all experimentally observed peaks. A direct comparison of the MP2/6-311G(2df,2pd) and experimental spectra suggests that the mode with the largest B-N stretching character is likely the experimentally observed peak at 608 cm-1, while M06-2X/6-311G( 2df ,2pd) frequencies suggest that this is the experimental peak at 568 cm-1. Although these results preclude a definitive assignment of the B-N stretching mode, the correlation between the calculated spectra and experimentally observed peaks strongly suggests that the vibrational mode dominated by the B-N stretching mode is one of four peaks in the range 560-650 cm -1.
This observation makes it one of the lowest, if not the lowest, energy vibrations observed to date and can be attributed mainly to B-N stretching.
Conclusions
Introduction
In 1996, Durig performed his ab intio study of phosphine boranes.28 He stated that there was no clear relationship between the chemical stability and bond lengths of the complexes he studied, BH3PH3, BH3PHF2, and BH3PF3. In 1998, Anane performed an ab initio molecular orbital study of the effect of substituents on various phosphine borane complexes using MP2/6-311G(d,p).29 They found that the bond length decreases with increasing methyl substitution on the phosphorus atom and increases with increasing substitution on the boron atom. He said that part of the difference may be due to the fact that the structural parameters of the borane moiety are not as well predicted as those of the methyl group due to its dative bond nature.
For example, BH3PMe3 has the largest bond dissociation energy of the complexes he studied, but does not have the shortest B-P bond length. In 2010, Janesko conducted a study on dative bond modeling in substituted boranes.34 In reviewing the history of the study, it was stated that B3LYP cannot reproduce accurate B–N bond lengths and bond dissociation energies for some species. Plumley and Evanseck found that B3LYP completely fails to predict B-N bond properties such as bond length and stretching frequency.35 Of the functionals tested by the authors, only the highly parametrized Minnesota functionals predict the experimental trends well.
We compare our experimental spectra with the results of ab initio calculations using the M06-2X method and basis sets of different sizes to fully describe the nature of the boron-phosphorus dative bond by examining the B-P stretching frequency in each case.
Spectroscopic Methods
Janesko used PBEsol GGA with either aug-cc-pVTZ or 6-311++G(3df,2p) to try and reproduce the accuracy of MPW1K and M06-2X at a lower computational cost.34 PBEsol GGA gives energies quite accurate and "reasonable, albeit redundant" geometries for a variety of ligands. In this study, we present experimental Raman spectra of B-P-containing molecules that have large functional groups. A scanning Raman HG2-S Raman spectrometer was used to acquired Raman spectra using the 514.5 nm line from an Ar ion laser.
A Nachet NS 400 micro-Raman setup was used to analyze the sample with an incident laser power of 300 mW on the sample, and a 0.5 cm.
Theoretical Methods
Results and Discussion
The systematic replacement of functional groups on the phosphorus atom has the effect of increasing the B-P stretching frequency with increasing total mass of the substituents. This effect is closely related to the degree of mixing between the B-P stretch and other normal modes localized on the substituents. To quantify this substitution effect, total energy distributions will be calculated for each molecule considered here and the degree of mixing within different molecular molecules.
The program INTDER2005 will be used to perform the TED for each of the molecules. These studies will help clarify where and what percentage of the B-P stretching frequency is in each different molecule, and based on how the molecule is substituted, a predictive trend of the physical properties of the dative B-P bond can be developed. Preliminary results indicate that molecules with substitution on the phosphorus atom have a much lower B-P stretching frequency than structures with substitution on the boron atom.
The substituent itself and the symmetry of the substitution also have significant effects on the location of the B-P stretch.
Conclusions
Poster Presentation Award, First Place, 2012 Mississippi State Meeting EPSCoR Tau Beta Pi Engineering Honor Society. University of Mississippi Outstanding Student in Chemical Engineering 2009 University of Mississippi Outstanding Junior in Chemical Engineering 2011 University of Mississippi Outstanding Student in Physical Chemistry 2011 University of Mississippi 2011 University of Mississippi 2011 ACS Outstanding Undergraduate Researcher at the University of Mississippi Sally McDonnell Barksdale Honors University of Mississippi College Chancellor's Honor Book. Hammer, "Detecting B-N and B-P Stretching Vibrations in Organic Molecules," Poster, 242nd National Meeting of the American Chemical Society, Denver, CO, August 2011.
