서울대학교_물리분석실험_NMR Spectroscopy(2024)

문서 내 토픽

1. NMR Spectroscopy

이번 실험에서는 NMR spectroscopy의 원리를 이해하고 2D-EXSY NMR을 통해 분자의 cis-trans insomerization reaction의 dynamic 성질을 파악하는 것을 목표로 한다. NMR(nuclear magnetic resonance)은 핵의 세차운동(precession)을 기본 원리로 한다. 원자는 자석과 같은 성질을 가지기 때문에 자기장을 걸어주면 세차운동이 발생한다. 따라서 핵스핀이 NMR 코일에 전력을 유도하게 되고, 각각 다른 속도로 세차운동하기 때문에 푸리에 변환(Fourier Transform, FT)을 하여 이 신호를 주파수 신호로 바꾼다. 이 주파수 신호는 chemical shift라고 하며 ppm단위를 갖는다. FT를 사용하면 감도와 해상도가 증가하기 때문에 다양한 차원의 NMR에 사용된다.
2. EXSY NMR

EXSY(exchange spectroscopy) NMR은 10ms~ 10s시간대의 dynamics를 측정하는 NMR로 반응의 time scale이 적절해야 한다. N,N-dimethylacetamide는 공명구조를 가지고 있기 때문에 cis-trans isomerization이 single bond보다는 느리고 double bond보다는 느려서 EXSY 방법으로 분석할 수 있는 분자적 특성을 가지고 있다. Mixing time을 주어 이 시간동안에 일어나는 isomerization에 따라 2D-NMR 신호에 기록된다.
3. Arrhenius Equation

Arrhenius equation을 통해 isomerization의 energy barrier도 구할 수 있는데, 이는 log scale에서 1/T와 lnk가 선형관계를 가지기 때문에 1/T의 기울기를 통하여 계산할 수 있다. 그렇게 계산한 energy barrier는 78.145kJ/mol로, 알려진 문헌값과 약 4%의 오차가 발생하였다.
4. Solvent Effects

이 오차는 실험마다 사용한 solvent의 종류와 농도에 따라 발생하거나 온도, 시간, NMR 장비의 전기적 noise등으로부터 기인했음을 추측할 수 있었고 d6-acetone의 전기적 특성이 CDCl3에 비해 EXSY NMR에 더 적합함을 알 수 있었다.
5. Reaction Kinetics

낮은 온도부터 rate constant는 각각 0.4870, 0.8264, 1.3650, 2.2064로, 온도가 증가할 수록 rate constant도 증가한다는 상관관계를 확인할 수 있었다. 같은 온도에서 mixing time이 증가할수록 diagonal peak는 감소하고 cross peak는 증가하였다. mixing time이 길어질수록 isomerization이 일어나는 횟수가 더 많아지기 때문이라고 해석할 수 있다.

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1. NMR Spectroscopy

NMR spectroscopy is a powerful analytical technique that provides valuable information about the structure and dynamics of molecules. It is widely used in various fields, including chemistry, biochemistry, and materials science. NMR spectroscopy relies on the magnetic properties of certain atomic nuclei, such as hydrogen (1H) and carbon (13C), to generate signals that can be analyzed to determine the chemical environment and connectivity of atoms within a molecule. The technique is non-destructive, allowing for the study of samples in their native state. NMR spectroscopy has undergone significant advancements in recent years, with the development of higher-field magnets, improved pulse sequences, and advanced data processing algorithms. These advancements have led to increased sensitivity, resolution, and the ability to study increasingly complex molecular systems. NMR spectroscopy is an indispensable tool in various fields, from organic chemistry and biochemistry to materials science and drug discovery, providing insights that are crucial for understanding the structure, function, and behavior of molecules.
2. EXSY NMR

EXSY (Exchange Spectroscopy) NMR is a powerful technique that allows the investigation of chemical exchange processes in molecules. It is particularly useful for studying dynamic systems, where atoms or functional groups within a molecule undergo reversible exchange reactions, such as conformational changes, tautomerization, or ligand binding events. EXSY NMR works by selectively exciting a specific set of nuclei and then monitoring the transfer of magnetization to other nuclei through the exchange process. By analyzing the resulting cross-peaks in the EXSY spectrum, researchers can obtain information about the rate and mechanism of the exchange process, as well as the connectivity and spatial relationships between the exchanging species. This technique is widely used in fields such as organic chemistry, biochemistry, and materials science, where understanding dynamic processes is crucial for elucidating the structure, function, and behavior of complex molecular systems. The development of advanced EXSY pulse sequences and the increasing availability of high-field NMR spectrometers have further expanded the applications of this technique, making it an indispensable tool for the study of complex molecular systems.
3. Arrhenius Equation

The Arrhenius equation is a fundamental relationship in chemical kinetics that describes the temperature dependence of reaction rates. It was developed by the Swedish chemist Svante Arrhenius in the late 19th century and has become a cornerstone of our understanding of chemical reactivity. The Arrhenius equation states that the rate constant of a chemical reaction is exponentially dependent on the activation energy of the reaction and the absolute temperature. This relationship is expressed mathematically as k = A * exp(-Ea/RT), where k is the rate constant, A is the pre-exponential factor, Ea is the activation energy, R is the universal gas constant, and T is the absolute temperature. The Arrhenius equation provides a simple and powerful way to predict how changes in temperature will affect the rate of a chemical reaction, which is crucial for understanding and optimizing a wide range of chemical processes, from industrial reactions to biological processes. The equation has found widespread applications in fields such as organic chemistry, biochemistry, materials science, and chemical engineering, and its continued use and refinement have been instrumental in advancing our understanding of the fundamental principles governing chemical reactivity.
4. Solvent Effects

Solvent effects are the influence of the surrounding solvent environment on the properties and behavior of chemical species. The choice of solvent can have a profound impact on the outcome of a chemical reaction, the stability and reactivity of molecules, and the spectroscopic properties of compounds. Solvent effects arise from a variety of intermolecular interactions, such as hydrogen bonding, dipole-dipole interactions, and solvation effects, which can stabilize or destabilize reaction intermediates, transition states, and products. Understanding and predicting solvent effects is crucial for optimizing chemical processes, designing new materials, and interpreting experimental data. Researchers in fields such as organic chemistry, biochemistry, and materials science have devoted significant effort to studying solvent effects, developing models and computational methods to account for these interactions. The ability to accurately predict and control solvent effects has led to advancements in areas like organic synthesis, catalysis, drug design, and the development of new functional materials. Continued research in this area is essential for expanding our understanding of the complex interplay between molecules and their surrounding environment, and for leveraging this knowledge to drive innovation in various scientific and technological domains.
5. Reaction Kinetics

Reaction kinetics is the study of the rates and mechanisms of chemical reactions, which is fundamental to our understanding of chemical processes and their optimization. The field of reaction kinetics encompasses the investigation of factors that influence the rate of a reaction, such as temperature, pressure, concentration of reactants, and the presence of catalysts or inhibitors. By studying the kinetics of a reaction, researchers can gain insights into the elementary steps involved, the energetics of the process, and the rate-limiting factors that determine the overall reaction rate. This knowledge is crucial for designing efficient chemical processes, predicting the behavior of complex systems, and developing new materials and technologies. Reaction kinetics has applications in a wide range of fields, including organic chemistry, biochemistry, materials science, and chemical engineering. Advances in experimental techniques, such as rapid-mixing methods and time-resolved spectroscopy, as well as the development of sophisticated computational models and simulations, have greatly expanded our ability to study and understand reaction kinetics at an increasingly detailed level. Continued research in this area is essential for addressing challenges in areas like energy conversion, environmental remediation, and the development of new drugs and functional materials, where a deep understanding of reaction kinetics is key to driving innovation and progress.