The 20th century is an era of masters, the beginning of the century quantum theory, relativity, wave-particle duality and other achievements in the physics revolution to promote a new look of chemistry, Pauling such a genius chemist born at the right time, research interests across chemistry, biochemistry, medicine and fruitful results, in addition to he is also a staunch pacifist, actively promote the anti-nuclear cause, won the reputation of "old man of peace", his life is wonderful, on the occasion of the 25th anniversary of Pauling's death, I would like to commemorate a generation of chemical masters with this article.
Born on February 18, 1901 in Portland, Oregon, on the west coast of the United States, Pauling's family was not well-off, and his father was a local pharmacist and the only source of income in the family, but his father died suddenly of illness when he was young, and the family's situation deteriorated sharply. Despite his hardships, Pauling was not demoralized, he continued to study while working to support his family, and showed a strong interest in chemistry.
In 1917, Pauling was admitted to the Department of Chemical Engineering of oregon Agricultural College, once dropped out of school due to family difficulties, and after successfully obtaining a bachelor's degree in 1922, he was admitted to the California Institute of Technology to follow the famous chemist Noyes to engage in crystal X-ray diffraction research, and successfully completed the full determination of mosoleum (mos2) crystals. In 1925, Pauling received a doctorate in philosophy in chemistry and successively engaged in research and research in several key laboratories in Europe, and it was at that time that Pauling came into contact with the emerging theory of quantum mechanics and modern physical testing methods, which laid an important foundation for his later proposal of chemical bond theory. Since then, Pauling has taught at his alma mater, the California Institute of Technology, and in 1969 became a professor of chemistry at Stanford University, and he has covered many fields and made mistakes in his life, but he cannot hide his glorious side in the history of science, and on August 19, 1994, Pauling died on his farm, completing his extraordinary life.
Figure 1 Pauling in his youth
Pauling's greatest contribution to chemistry is his study of the nature of chemical bonds and their application in the structure of matter, which he has been very interested in since his youth. Shortly after physicists modeled the structure of atoms, the American chemist G. n. lewis) proposed an electron model of a "shared electron pair" to achieve a stable structure of a noble gas, and a few years later Langmuir (i. Langmuir accepted and developed Lewis's view, proposing to use "covalent bonds" to represent a common pair of electrons. Yet they still can't scientifically explain the nature of chemical bonds, that is, why atoms choose to "share" electrons, and what is the "force" that firmly binds electrons that should be mutually exclusive?
Figure 2 American chemist g. n. lewis and i. langmuir
Limited by the old quantum theory, Louis-Langmuir's covalent bond theory is essentially a static model, not only unable to elucidate the nature of bonding, but also unable to explain the unique spectral phenomena of hydrogen molecules (h2). In 1927, the German chemist W. Hettler (W. Hettler) h. heitler) and London (f. w. london) creatively applied quantum mechanical methods to the treatment of hydrogen molecules, marking the birth of quantum chemistry and laying the foundation for the near-cost bond theory. The results of Hettler-London's calculations show that due to the fluctuation of electrons, the interference of inter-atom waves increases the electron density in the overlapping regions of atomic orbits, and single electrons with opposite spins (unpaired electrons) show mutual attraction to each other in the process of mutual proximity, and reduce the energy of the system, which also solves the essential problem of chemical bonds.
Figure 3 German chemist w. h. heitler and f. w. london
The above theory is called valence bond theory, commonly known as the vb method, and is often referred to as the electron pairing theory. It has been very successful in explaining the directionality and saturation of covalent bonds and in qualitatively discussing molecular structure, but the imperfections of the early stages of the theory are also revealed, such as the bond angle of some molecules is significantly deviated from the angle between atomic orbitals, and the number of bonds of some atoms is greater than the number of unpaired electrons in the valence layer orbital, and the vb method appears powerless to explain these phenomena.
In order to explain the spatial structure of polyatomic molecules, Pauling creatively proposed the hybrid orbital theory (hybrid orbital theory) on the basis of the vb method in 1931, which reasonably explained the tetrahedral configuration of methane molecules (ch4), further enriching and developing the vb theory. In order to measure the attractance of atoms in compounds to "bonded electron pairs (bonded electrons)", Pauling also took the lead in proposing the concept of "electronegativity", which was based on thermochemical and bond energy data, and systematically gave electronegativity scale data, which are still widely used today and play an important role in predicting the degree of ionicity or covalentity of compounds and chemical bonds.
Figure 4 Hybrid orbital theory explains the bonding of methane molecules
In addition to the above contributions, it should also be noted that Pauling is the founder of the "resonance theory". Many molecules, such as benzene (c6h6) and ozone (o3) molecules, need two or more valence bond structures to give a satisfactory description, so Pauling believes that it is the "resonance" between these valence bond structures that completely constitutes the true structure of the molecule. However, in the decades after the advent of the "resonance theory", there was much debate and mixed praise, and in the early 1950s, Soviet scholars made a philosophical and erroneous criticism of it from the philosophical aspect of "idealism" and "mechanism", which also affected the later Chinese academic circles. From the perspective of modern chemistry, "resonance theory" does have its drawbacks, but after all, it is more concise and intuitive than the mainstream molecular orbital theory, and it is still practical in qualitatively explaining some properties of compounds, so it has not completely withdrawn from the historical stage, and "resonance theory" is still widely used in textbooks and literature works at home and abroad.
Figure 5 "resonance" structure of benzene and nitrogen dioxide molecules
Pauling began to devote himself to the study of chemical bonds in the 1930s, published valence bond theory in February 1931, and has since published related papers, and in 1939 published the epoch-making book "The Nature of Chemical Bonds" in the history of chemistry. The book revolutionized the understanding of chemical bonds, elevating them from intuitive, imaginary concepts to quantitative and rational levels, and pauling won the 1954 Nobel Prize in Chemistry for his outstanding contributions to the nature of chemical bonds and the interpretation of the structure of complex compound matter.
Pauling's main research content in the early period was chemical bond theory, and from the middle to late stages, his research field gradually expanded to the field of biology and medicine. Since 1937, he has begun to study the structure of amino acids and proteins, and formally determined the a-heponemal structure of proteins - this is a major breakthrough in the field of protein research, and also provides a theoretical basis for later determination of DNA structure. Although Pauling ultimately failed to properly reveal the double helix structure of DNA, and had a debate with biologists Watson and Crick about the true structure of DNA, it is impossible to deny that he laid an important foundation for subsequent protein structure research.
Figure 6 A-spirochetes structure of proteins (left), dna structure discoverers Watson and Crick (middle), and spiral structures of dna and RNA (right) (image from the network)
In 1945, Pauling began his research on "molecular diseases", when it was widely believed that sickle cell anemia was just a typical cell type disease caused by the deformation of red blood cells, but Pauling was keenly aware that the disease was most likely a disease of hemoglobin molecules. To this end, he successfully discovered the difference in the migration rate of normal and abnormal hemoglobin in the same electric field with the help of electrophoresis technology, and in November 1949 Pauling published a paper in science, which discussed in detail the difference between abnormal hemoglobin and normal hemoglobin, and discussed the causes and genetic mechanisms of the disease. Pauling's research on sickle-type cell hemoglobin showed the molecular basis of this disease for the first time, and it was also the first time to put forward the concept of "molecular disease" in the true sense, attracting follow-up medical researchers to conduct disease research at the molecular level.
Figure 7 Normal hemoglobin (round cake) and sickle cell anemia abnormal hemoglobin (image from the network)
In the 1950s, shortly after the end of World War II, Pauling paid special attention to the issue of war and peace around the world. At that time, the world was conducting frequent nuclear tests, and Pauling realized that nuclear radiation posed a huge threat to human survival and health, and in 1955 he joined forces with Einstein and other scientists to oppose the research and production of destructive weapons. In 1958, he wrote the book "No More War", which explained the great threat of nuclear weapons to mankind with rich information. In 1962, Pauling was awarded the Nobel Peace Prize for his efforts against nuclear weapons testing, becoming the second scientist after Marie Curie to receive a different Nobel Prize, and the only one to win an independent prize each time.
Figure 8 Pauling's Book of No More Wars
In the 1960s, there was a sudden vitamin boom in the United States, and the creator of this boom was Pauling. He believes that vitamin C can strengthen the body's immune system and has a significant effect on colds, and he pointed out that taking large doses of vitamin C can also alleviate cancer. To this end, he did a lot of research and publicity work, people are convinced of this scientist, and vitamin C suddenly became a "star molecule". In addition, Pauling also believes that vitamin C is a common "orthomolecular" and hypes the so-called "orthomolecular medicine", but this view seriously challenges the understanding of traditional medicine, is considered a threat to traditional medicine, and has never been recognized by the mainstream medical community to this day.
Figure 9 Commercialized Vitamin C and Its Structural Formula (Image from the Web)
Although Pauling's claims have been enthusiastically pursued by the general public, they have not been recognized by the medical and nutritional circles, and the voices of opposition and criticism are endless, and the former "scientific giants" have also been attacked as "quackery doctors", and he is also regarded as a "pseudo-science communicator". To this day, the controversy over Pauling's strong promotion of vitamin C in his later years has not completely receded, and there is no consensus on the health effects of vitamin C and the dosage of vitamin C. Objectively speaking, regardless of the specific focus of the controversy, the important role of vitamins on living organisms is still one of the important discoveries of the 20th century, which is unquestionable and indisputable.
Pauling's lifelong contributions to chemistry are quite a lot, the impact on the young generation of chemists is also extremely far-reaching, he is very concerned about China's chemical undertakings, in 1973 and 1981 twice to China's academic visits and exchanges, the famous chemists Tang Youqi and Mr. Lu Jiaxi have studied and studied under Pauling's guidance.
Evaluating Pauling from the process of modernization, he creatively put forward the most basic and widely used concepts in the field of chemistry such as hybrid orbit, electronegativity, and resonance theory, greatly enriched and developed the valence bond theory, and became a well-deserved "founder of modern structural chemistry"; across the fields of biology and medicine, his research pointed out the direction for future generations; for world peace, the humanistic spirit of scientists was excellently interpreted in him. Despite the controversy in his later years, Pauling's spirit of daring to be a pioneer and actively exploring new areas still deserves our appreciation. As a highly creative scientist and social activist with a passion for world peace, Pauling perfectly interprets the qualities that a scientist should have, and this quality will be passed on from generation to generation, influencing and inspiring generations of scientists.
Resources:
Sheng Genyu. Pauling, explorer of the nature of chemical bonds[j]. Teaching Chemistry, 2011(11): 57-60.
ZHANG Hongzhi, SHAO Lijian. Pauling's contribution to hemoglobin molecular science[j]. University Chemistry, 2012, 27(6).
Chu Tingfu. Pauling, known as a "first-class genius" and a "peaceful old man"[j]. Teaching Chemistry, 1998, 6: 10-11.
Tian Hezhen. Pauling and Modern chemistry[j]. University Chemistry, 1987, 2(2): 56-59.
Zhou Gongdu, Duan Lianyun. Fundamentals of Structural Chemistry (5th Edition)[m]. Peking University Press, 2017
Huaisha. Vitamin controversy[j]. Life World, 2010(3): 26-31.
Lü Renqing et al. Pauling and resonance theory- Controversy over resonance theory[j]. Guangdong Chemical Industry, 2010, 37(5): 23-24.
Written by Geronimo