Thermochemistry
Thermochemistry is the study of the heat energy associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes on the system's energy exchange with its surroundings. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. In combination with entropy determinations, it is used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable. Endothermic reactions absorb heat. Thermochemistry coalesces the concepts of thermodynamics with the concept of energy in the form of chemical bonds; the subject includes calculations of such quantities as heat capacity, heat of combustion, heat of formation, entropy, free energy, calories. Thermochemistry rests on two generalizations. Stated in modern terms, they are as follows: Lavoisier and Laplace's law: The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process.
Hess' law: The energy change accompanying any transformation is the same whether the process occurs in one step or many. These statements helped in its formulation. Lavoisier and Hess investigated specific heat and latent heat, although it was Joseph Black who made the most important contributions to the development of latent energy changes. Gustav Kirchhoff showed in 1858 that the variation of the heat of reaction is given by the difference in heat capacity between products and reactants: dΔH / dT = ΔCp. Integration of this equation permits the evaluation of the heat of reaction at one temperature from measurements at another temperature; the measurement of heat changes is performed using calorimetry an enclosed chamber within which the change to be examined occurs. The temperature of the chamber is monitored either using a thermometer or thermocouple, the temperature plotted against time to give a graph from which fundamental quantities can be calculated. Modern calorimeters are supplied with automatic devices to provide a quick read-out of information, one example being the differential scanning calorimeter.
Several thermodynamic definitions are useful in thermochemistry. A system is the specific portion of the universe, being studied. Everything outside the system is considered environment. A system may be: a isolated system which can exchange neither energy nor matter with the surroundings, such as an insulated bomb calorimeter a thermally isolated system which can exchange mechanical work but not heat or matter, such as an insulated closed piston or balloon a mechanically isolated system which can exchange heat but not mechanical work or matter, such as an uninsulated bomb calorimeter a closed system which can exchange energy but not matter, such as an uninsulated closed piston or balloon an open system which it can exchange both matter and energy with the surroundings, such as a pot of boiling water A system undergoes a process when one or more of its properties changes. A process relates to the change of state. An isothermal process occurs. An isobaric process occurs. A process is adiabatic.
Differential scanning calorimetry Important publications in thermochemistry Isodesmic reaction Principle of maximum work Reaction Calorimeter Thomsen-Berthelot principle Julius Thomsen Thermodynamic databases for pure substances Calorimetry Photoelectron photoion coincidence spectroscopy Thermodynamics Cryochemistry Chemical kinetics "Thermochemistry". Encyclopædia Britannica. 26. 1911. Pp. 804–808
Tetrathiafulvalene
Tetrathiafulvalene is an organosulfur compound with the formula 2. Studies on this heterocyclic compound contributed to the development of molecular electronics. TTF is related to 2, by replacement of four CH groups with sulfur atoms. Over 10,000 scientific publications discuss its derivatives; the high level of interest in TTFs has spawned the development of many syntheses of TTF and its analogues. Most preparations entail the coupling of cyclic C3S2 building blocks such as 1,3-dithiole-2-thiones or the related 1,3-dithiole-2-ones. For TTF itself, the synthesis begins with the trithiocarbonate H2C2S2CS, S-methylated and reduced to give H2C2S2CH, treated as follows: H2C2S2CH + HBF4 → BF−4 + HSCH32 BF−4 + 2 Et3N → 2 + 2 Et3NHBF4 Bulk TTF itself has unremarkable electrical properties. Distinctive properties are, associated with salts of its oxidized derivatives, such as salts derived from TTF+; the high electrical conductivity of TTF salts can be attributed to the following features of TTF: its planarity, which allows π-π stacking of its oxidized derivatives, its high symmetry, which promotes charge delocalization, thereby minimizing coulombic repulsions, its ability to undergo oxidation at mild potentials to give a stable radical cation.
Electrochemical measurements show that TTF can be oxidized twice reversibly: TTF → TTF+ + e− TTF+ → TTF2+ + e− Each dithiolylidene ring in TTF has 7π electrons: 2 for each sulfur atom, 1 for each sp2 carbon atom. Thus, oxidation converts each ring to an aromatic 6π-electron configuration leaving the central double bond a single bond, as all π-electrons occupy ring orbitals; the salt Cl− was reported to be a semiconductor in 1972. Subsequently, the charge-transfer salt TCNQ was shown to be a narrow band gap semiconductor. X-ray diffraction studies of revealed stacks of oxidized TTF molecules adjacent to anionic stacks of TCNQ molecules; this “segregated stack” motif was unexpected and is responsible for the distinctive electrical properties, i.e. high and anisotropic electrical conductivity. Since these early discoveries, numerous analogues of TTF have been prepared. Well studied analogues include tetramethyltetrathiafulvalene, tetramethylselenafulvalenes, bistetrathiafulvalene. Several tetramethyltetrathiafulvalene salts are of some relevance as organic superconductors.
Bechgaard salt Rovira, C.. "Bistetrathiafulvalene and Related Dissymmetrical Electron Donors: From the Molecule to Functional Molecular Materials and Devices". Chemical Reviews. 104: 5289–5317. Doi:10.1021/cr030663+. PMID 15535651. Iyoda, M. "Bi-TTF, Bis-TTF, Related TTF Oligomers". Chemical Reviews. 104: 5085–5113. Doi:10.1021/cr030651o. PMID 15535643. Frere, P.. J.. "Salts of Extended Tetrathiafulvalene analogues: relationships Between Molecular Structure, Electrochemical Properties and Solid State Organization". Chemical Society Reviews. 34: 69–98. Doi:10.1039/b316392j. PMID 15643491. Gorgues, Alain. "Highly Functionalized Tetrathiafulvalenes: Riding along the Synthetic Trail from Electrophilic Alkynes". Chemical Reviews. 104: 5151–5184. Doi:10.1021/cr0306485. PMID 15535646. Physical properties of Tetrathiafulvalene from the literature
Solid-state physics
Solid-state physics is the study of rigid matter, or solids, through methods such as quantum mechanics, crystallography and metallurgy. It is the largest branch of condensed matter physics. Solid-state physics studies how the large-scale properties of solid materials result from their atomic-scale properties. Thus, solid-state physics forms a theoretical basis of materials science, it has direct applications, for example in the technology of transistors and semiconductors. Solid materials are formed from densely packed atoms; these interactions produce the mechanical, electrical and optical properties of solids. Depending on the material involved and the conditions in which it was formed, the atoms may be arranged in a regular, geometric pattern or irregularly; the bulk of solid-state physics, as a general theory, is focused on crystals. This is because the periodicity of atoms in a crystal — its defining characteristic — facilitates mathematical modeling. Crystalline materials have electrical, optical, or mechanical properties that can be exploited for engineering purposes.
The forces between the atoms in a crystal can take a variety of forms. For example, in a crystal of sodium chloride, the crystal is made up of ionic sodium and chlorine, held together with ionic bonds. In others, the atoms share form covalent bonds. In metals, electrons are shared amongst the whole crystal in metallic bonding; the noble gases do not undergo any of these types of bonding. In solid form, the noble gases are held together with van der Waals forces resulting from the polarisation of the electronic charge cloud on each atom; the differences between the types of solid result from the differences between their bonding. The physical properties of solids have been common subjects of scientific inquiry for centuries, but a separate field going by the name of solid-state physics did not emerge until the 1940s, in particular with the establishment of the Division of Solid State Physics within the American Physical Society; the DSSP catered to industrial physicists, solid-state physics became associated with the technological applications made possible by research on solids.
By the early 1960s, the DSSP was the largest division of the American Physical Society. Large communities of solid state physicists emerged in Europe after World War II, in particular in England and the Soviet Union. In the United States and Europe, solid state became a prominent field through its investigations into semiconductors, superconductivity, nuclear magnetic resonance, diverse other phenomena. During the early Cold War, research in solid state physics was not restricted to solids, which led some physicists in the 1970s and 1980s to found the field of condensed matter physics, which organized around common techniques used to investigate solids, liquids and other complex matter. Today, solid-state physics is broadly considered to be the subfield of condensed matter physics that focuses on the properties of solids with regular crystal lattices. Many properties of materials are affected by their crystal structure; this structure can be investigated using a range of crystallographic techniques, including X-ray crystallography, neutron diffraction and electron diffraction.
The sizes of the individual crystals in a crystalline solid material vary depending on the material involved and the conditions when it was formed. Most crystalline materials encountered in everyday life are polycrystalline, with the individual crystals being microscopic in scale, but macroscopic single crystals can be produced either or artificially. Real crystals feature defects or irregularities in the ideal arrangements, it is these defects that critically determine many of the electrical and mechanical properties of real materials. Properties of materials such as electrical conduction and heat capacity are investigated by solid state physics. An early model of electrical conduction was the Drude model, which applied kinetic theory to the electrons in a solid. By assuming that the material contains immobile positive ions and an "electron gas" of classical, non-interacting electrons, the Drude model was able to explain electrical and thermal conductivity and the Hall effect in metals, although it overestimated the electronic heat capacity.
Arnold Sommerfeld combined the classical Drude model with quantum mechanics in the free electron model. Here, the electrons are modelled as a Fermi gas, a gas of particles which obey the quantum mechanical Fermi–Dirac statistics; the free electron model gave improved predictions for the heat capacity of metals, however, it was unable to explain the existence of insulators. The nearly free electron model is a modification of the free electron model which includes a weak periodic perturbation meant to model the interaction between the conduction electrons and the ions in a crystalline solid. By introducing the idea of electronic bands, the theory explains the existence of conductors and insulators; the nearly free electron model rewrites the Schrödinger equation for the case of a periodic potential. The solutions in this case are known as Bloch states. Since Bloch's theorem applies only to periodic potentials, since unceasing random movements of atoms in a crystal disrupt periodicity, this use of Bloch's theorem is only an approximation, but it has proven to be a tremendously valuable approximation, without which most solid-state physics analysis would be intractable.
Deviations from periodici
Platinum
Platinum is a chemical element with symbol Pt and atomic number 78. It is a dense, ductile unreactive, silverish-white transition metal, its name is derived from the Spanish term platino, meaning "little silver". Platinum is a member of the platinum group of elements and group 10 of the periodic table of elements, it has six occurring isotopes. It is one of the rarer elements in Earth's crust, with an average abundance of 5 μg/kg, it occurs in some nickel and copper ores along with some native deposits in South Africa, which accounts for 80% of the world production. Because of its scarcity in Earth's crust, only a few hundred tonnes are produced annually, given its important uses, it is valuable and is a major precious metal commodity. Platinum is one of the least reactive metals, it has remarkable resistance to corrosion at high temperatures, is therefore considered a noble metal. Platinum is found chemically uncombined as native platinum; because it occurs in the alluvial sands of various rivers, it was first used by pre-Columbian South American natives to produce artifacts.
It was referenced in European writings as early as 16th century, but it was not until Antonio de Ulloa published a report on a new metal of Colombian origin in 1748 that it began to be investigated by scientists. Platinum is used in catalytic converters, laboratory equipment, electrical contacts and electrodes, platinum resistance thermometers, dentistry equipment, jewelry. Being a heavy metal, it leads to health problems upon exposure to its salts. Compounds containing platinum, such as cisplatin and carboplatin, are applied in chemotherapy against certain types of cancer; as of 2018, the value of platinum is $833.00 per ounce. Pure platinum is a lustrous and malleable, silver-white metal. Platinum is more ductile than gold, silver or copper, thus being the most ductile of pure metals, but it is less malleable than gold; the metal has excellent resistance to corrosion, is stable at high temperatures and has stable electrical properties. Platinum does oxidize, forming PtO2, at 500 °C, it reacts vigorously with fluorine at 500 °C to form platinum tetrafluoride.
It is attacked by chlorine, bromine and sulfur. Platinum is insoluble in hydrochloric and nitric acid, but dissolves in hot aqua regia, to form chloroplatinic acid, H2PtCl6, its physical characteristics and chemical stability make it useful for industrial applications. Its resistance to wear and tarnish is well suited to use in fine jewellery; the most common oxidation states of platinum are +2 and +4. The +1 and +3 oxidation states are less common, are stabilized by metal bonding in bimetallic species; as is expected, tetracoordinate platinum compounds tend to adopt 16-electron square planar geometries. Although elemental platinum is unreactive, it dissolves in hot aqua regia to give aqueous chloroplatinic acid: Pt + 4 HNO3 + 6 HCl → H2PtCl6 + 4 NO2 + 4 H2OAs a soft acid, platinum has a great affinity for sulfur, such as on dimethyl sulfoxide. In 2007, Gerhard Ertl won the Nobel Prize in Chemistry for determining the detailed molecular mechanisms of the catalytic oxidation of carbon monoxide over platinum.
Platinum has six occurring isotopes: 190Pt, 192Pt, 194Pt, 195Pt, 196Pt, 198Pt. The most abundant of these is 195 Pt, it is the only stable isotope with a non-zero spin. 190Pt is the least abundant at only 0.01%. Of the occurring isotopes, only 190Pt is unstable, though it decays with a half-life of 6.5×1011 years, causing an activity of 15 Bq/kg of natural platinum. 198 Pt can undergo alpha decay. Platinum has 31 synthetic isotopes ranging in atomic mass from 166 to 204, making the total number of known isotopes 39; the least stable of these is 166Pt, with a half-life of 300 µs, whereas the most stable is 193Pt with a half-life of 50 years. Most platinum isotopes decay by some combination of beta alpha decay. 188Pt, 191Pt, 193Pt decay by electron capture. 190Pt and 198Pt are predicted to have energetically favorable double beta decay paths. Platinum is an rare metal, occurring at a concentration of only 0.005 ppm in Earth's crust. It is sometimes mistaken for silver. Platinum is found chemically uncombined as native platinum and as alloy with the other platinum-group metals and iron mostly.
Most the native platinum is found in secondary deposits in alluvial deposits. The alluvial deposits used by pre-Columbian people in the Chocó Department, Colombia are still a source for platinum-group metals. Another large alluvial deposit is in the Ural Mountains, it is still mined. In nickel and copper deposits, platinum-group metals occur as sulfides, tellurides and arsenides, as end alloys with nickel or copper. Platinum arsenide, sperrylite, is a major source of platinum associated with nickel ores in the Sudbury Basin deposit in Ontario, Canada. At Platinum, about 17,000 kg was mined between 1927 and 1975; the mine ceased operations in 1990. The rare sulfide minera
Organic chemistry
Organic chemistry is a subdiscipline of chemistry that studies the structure and reactions of organic compounds, which contain carbon in covalent bonding. Study of structure determines their chemical formula. Study of properties includes physical and chemical properties, evaluation of chemical reactivity to understand their behavior; the study of organic reactions includes the chemical synthesis of natural products and polymers, study of individual organic molecules in the laboratory and via theoretical study. The range of chemicals studied in organic chemistry includes hydrocarbons as well as compounds based on carbon, but containing other elements oxygen, sulfur and the halogens. Organometallic chemistry is the study of compounds containing carbon–metal bonds. In addition, contemporary research focuses on organic chemistry involving other organometallics including the lanthanides, but the transition metals zinc, palladium, cobalt and chromium. Organic compounds constitute the majority of known chemicals.
The bonding patterns of carbon, with its valence of four—formal single and triple bonds, plus structures with delocalized electrons—make the array of organic compounds structurally diverse, their range of applications enormous. They form the basis of, or are constituents of, many commercial products including pharmaceuticals; the study of organic chemistry overlaps organometallic chemistry and biochemistry, but with medicinal chemistry, polymer chemistry, materials science. Before the nineteenth century, chemists believed that compounds obtained from living organisms were endowed with a vital force that distinguished them from inorganic compounds. According to the concept of vitalism, organic matter was endowed with a "vital force". During the first half of the nineteenth century, some of the first systematic studies of organic compounds were reported. Around 1816 Michel Chevreul started a study of soaps made from various alkalis, he separated the different acids. Since these were all individual compounds, he demonstrated that it was possible to make a chemical change in various fats, producing new compounds, without "vital force".
In 1828 Friedrich Wöhler produced the organic chemical urea, a constituent of urine, from inorganic starting materials, in what is now called the Wöhler synthesis. Although Wöhler himself was cautious about claiming he had disproved vitalism, this was the first time a substance thought to be organic was synthesized in the laboratory without biological starting materials; the event is now accepted as indeed disproving the doctrine of vitalism. In 1856 William Henry Perkin, while trying to manufacture quinine accidentally produced the organic dye now known as Perkin's mauve, his discovery, made known through its financial success increased interest in organic chemistry. A crucial breakthrough for organic chemistry was the concept of chemical structure, developed independently in 1858 by both Friedrich August Kekulé and Archibald Scott Couper. Both researchers suggested that tetravalent carbon atoms could link to each other to form a carbon lattice, that the detailed patterns of atomic bonding could be discerned by skillful interpretations of appropriate chemical reactions.
The era of the pharmaceutical industry began in the last decade of the 19th century when the manufacturing of acetylsalicylic acid—more referred to as aspirin—in Germany was started by Bayer. By 1910 Paul Ehrlich and his laboratory group began developing arsenic-based arsphenamine, as the first effective medicinal treatment of syphilis, thereby initiated the medical practice of chemotherapy. Ehrlich popularized the concepts of "magic bullet" drugs and of systematically improving drug therapies, his laboratory made decisive contributions to developing antiserum for diphtheria and standardizing therapeutic serums. Early examples of organic reactions and applications were found because of a combination of luck and preparation for unexpected observations; the latter half of the 19th century however witnessed systematic studies of organic compounds. The development of synthetic indigo is illustrative; the production of indigo from plant sources dropped from 19,000 tons in 1897 to 1,000 tons by 1914 thanks to the synthetic methods developed by Adolf von Baeyer.
In 2002, 17,000 tons of synthetic indigo were produced from petrochemicals. In the early part of the 20th century and enzymes were shown to be large organic molecules, petroleum was shown to be of biological origin; the multiple-step synthesis of complex organic compounds is called total synthesis. Total synthesis of complex natural compounds increased in complexity to terpineol. For example, cholesterol-related compounds have opened ways to synthesize complex human hormones and their modified derivatives. Since the start of the 20th century, complexity of total syntheses has been increased to include molecules of high complexity such as lysergic acid and vitamin B12; the discovery of petroleum and the development of the petrochemical industry spurred the development of organic chemistry. Converting individual petroleum compounds into different types of compounds by various chemical processes led to organic reactions enabling a broad range of
Surface science
Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid–liquid interfaces, solid–gas interfaces, solid–vacuum interfaces, liquid–gas interfaces. It includes the fields of surface surface physics; some related practical applications are classed as surface engineering. The science encompasses concepts such as heterogeneous catalysis, semiconductor device fabrication, fuel cells, self-assembled monolayers, adhesives. Surface science is related to interface and colloid science. Interfacial chemistry and physics are common subjects for both; the methods are different. In addition and colloid science studies macroscopic phenomena that occur in heterogeneous systems due to peculiarities of interfaces; the field of surface chemistry started with heterogeneous catalysis pioneered by Paul Sabatier on hydrogenation and Fritz Haber on the Haber process. Irving Langmuir was one of the founders of this field, the scientific journal on surface science, bears his name.
The Langmuir adsorption equation is used to model monolayer adsorption where all surface adsorption sites have the same affinity for the adsorbing species and do not interact with each other. Gerhard Ertl in 1974 described for the first time the adsorption of hydrogen on a palladium surface using a novel technique called LEED. Similar studies with platinum and iron followed. Most recent developments in surface sciences include the 2007 Nobel prize of Chemistry winner Gerhard Ertl's advancements in surface chemistry his investigation of the interaction between carbon monoxide molecules and platinum surfaces. Surface chemistry can be defined as the study of chemical reactions at interfaces, it is related to surface engineering, which aims at modifying the chemical composition of a surface by incorporation of selected elements or functional groups that produce various desired effects or improvements in the properties of the surface or interface. Surface science is of particular importance to the fields of heterogeneous catalysis and geochemistry.
The adhesion of gas or liquid molecules to the surface is known as adsorption. This can be due to either chemisorption or physisorption, the strength of molecular adsorption to a catalyst surface is critically important to the catalyst's performance. However, it is difficult to study these phenomena in real catalyst particles, which have complex structures. Instead, well-defined single crystal surfaces of catalytically active materials such as platinum are used as model catalysts. Multi-component materials systems are used to study interactions between catalytically active metal particles and supporting oxides. Relationships between the composition and chemical behavior of these surfaces are studied using ultra-high vacuum techniques, including adsorption and temperature-programmed desorption of molecules, scanning tunneling microscopy, low energy electron diffraction, Auger electron spectroscopy. Results can be used toward the rational design of new catalysts. Reaction mechanisms can be clarified due to the atomic-scale precision of surface science measurements.
Electrochemistry is the study of processes driven through an applied potential at a solid-liquid or liquid-liquid interface. The behavior of an electrode-electrolyte interface is affected by the distribution of ions in the liquid phase next to the interface forming the electrical double layer. Adsorption and desorption events can be studied at atomically flat single crystal surfaces as a function of applied potential and solution conditions using spectroscopy, scanning probe microscopy and surface X-ray scattering; these studies link traditional electrochemical techniques such as cyclic voltammetry to direct observations of interfacial processes. Geologic phenomena such as iron cycling and soil contamination are controlled by the interfaces between minerals and their environment; the atomic-scale structure and chemical properties of mineral-solution interfaces are studied using in situ synchrotron X-ray techniques such as X-ray reflectivity, X-ray standing waves, X-ray absorption spectroscopy as well as scanning probe microscopy.
For example, studies of heavy metal or actinide adsorption onto mineral surfaces reveal molecular-scale details of adsorption, enabling more accurate predictions of how these contaminants travel through soils or disrupt natural dissolution-precipitation cycles. Surface physics can be defined as the study of physical interactions that occur at interfaces, it overlaps with surface chemistry. Some of the topics investigated in surface physics include friction, surface states, surface diffusion, surface reconstruction, surface phonons and plasmons, the emission and tunneling of electrons and the self-assembly of nanostructures on surfaces. Techniques to investigate processes at surfaces include Surface X-Ray Scattering, Scanning Probe Microscopy, surface enhanced Raman Spectroscopy and X-ray Photoelectron Spectroscopy; the study and analysis of surfaces involves both chemical analysis techniques. Several modern methods probe the topmost 1–10 nm of surfaces exposed to vacuum; these include X-ray photoelectron spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, electron energy loss spectroscopy, thermal desorption spectroscopy, ion scattering spectroscopy, secondary ion mass spectrometry, dual polarization interferometry, other surface analysis methods included in the list of materials analysis methods.
Many of these techniques require vacuum as they rely on the detection of ele
Photochemistry
Photochemistry is the branch of chemistry concerned with the chemical effects of light. This term is used to describe a chemical reaction caused by absorption of ultraviolet, visible light or infrared radiation. In nature, photochemistry is of immense importance as it is the basis of photosynthesis and the formation of vitamin D with sunlight. Photochemical reactions proceed differently than temperature-driven reactions. Photochemical paths access high energy intermediates that cannot be generated thermally, thereby overcoming large activation barriers in a short period of time, allowing reactions otherwise inaccessible by thermal processes. Photochemistry is destructive, as illustrated by the photodegradation of plastics. Photoexcitation is the first step in a photochemical process where the reactant is elevated to a state of higher energy, an excited state; the first law of photochemistry, known as the Grotthuss–Draper law, states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place.
According to the second law of photochemistry, known as the Stark-Einstein law, for each photon of light absorbed by a chemical system, no more than one molecule is activated for a photochemical reaction, as defined by the quantum yield. When a molecule or atom in the ground state absorbs light, one electron is excited to a higher orbital level; this electron maintains its spin according to the spin selection rule. The excitation to a higher singlet state can be from HOMO to LUMO or to a higher orbital, so that singlet excitation states S1, S2, S3… at different energies are possible. Kasha's rule stipulates that higher singlet states would relax by radiationless decay or internal conversion to S1. Thus, S1 is but not always, the only relevant singlet excited state; this excited state S1 can further relax to S0 by IC, but by an allowed radiative transition from S1 to S0 that emits a photon. Alternatively, it is possible for the excited state S1 to undergo spin inversion and to generate a triplet excited state T1 having two unpaired electrons with the same spin.
This violation of the spin selection rule is possible by intersystem crossing of the vibrational and electronic levels of S1 and T1. According to Hund's rule of maximum multiplicity, this T1 state would be somewhat more stable than S1; this triplet state can relax to the ground state S0 by radiationless IC or by a radiation pathway called phosphorescence. This process implies a change of electronic spin, forbidden by spin selection rules, making phosphorescence much slower than fluorescence. Thus, triplet states have longer lifetimes than singlet states; these transitions are summarized in a state energy diagram or Jablonski diagram, the paradigm of molecular photochemistry. These excited species, either S1 or T1, have a half empty low-energy orbital, are more oxidizing than the ground state, but at the same time, they have an electron in a high energy orbital, are thus more reducing. In general, excited species are prone to participate in electron transfer processes. Photochemical reactions require a light source that emits wavelengths corresponding to an electronic transition in the reactant.
In the early experiments, sunlight was the light source. Mercury-vapor lamps are more common in the laboratory. Low pressure mercury vapor lamps emit at 254 nm. For polychromatic sources, wavelength ranges can be selected using filters. Alternatively, laser beams are monochromatic and LEDs have a narrowband that can be efficiently used, as well as Rayonet lamps, to get monochromatic beams; the emitted light must of course reach the targeted functional group without being blocked by the reactor, medium, or other functional groups present. For many applications, quartz is used for the reactors as well as to contain the lamp. Pyrex absorbs at wavelengths shorter than 275 nm; the solvent is an important experimental parameter. Solvents are potential reactants and for this reason, chlorinated solvents are avoided because the C-Cl bond can lead to chlorination of the substrate. Absorbing solvents prevent photons from reaching the substrate. Hydrocarbon solvents absorb only at short wavelengths and are thus preferred for photochemical experiments requiring high energy photons.
Solvents containing unsaturation absorb at longer wavelengths and can usefully filter out short wavelengths. For example and acetone "cut off" at wavelengths shorter than 215 and 330 nm, respectively. Continuous flow photochemistry offers multiple advantages over batch photochemistry. Photochemical reactions are driven by the number of photons that are able to activate molecules causing the desired reaction; the large surface area to volume ratio of a microreactor maximizes the illumination, at the same time allows for efficient cooling, which decreases the thermal side products. In the case of photochemical reactions, light provides the activation energy. Simplistically, light is one mechanism for providing the activation energy required for many reactions. If laser light is employed, it is possible to selectively excite a molecule so as to produce a desired electronic and vibrational state; the emission from a particular state may be selectively monitored, providing a measure of the population of that state
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