AST03 - Astrochemistry PDF

AST03 - Astrochemistry Terms to Remember Astrochemistry - abundance and reactions of molecules in the Universe and their interaction with radiation. Cosmochemistry - abundance of elements, isotope ratios in the Solar System (meteorites) Molecular Astrophysics - interste...

AST03 - Astrochemistry Terms to Remember Astrochemistry - abundance and reactions of molecules in the Universe and their interaction with radiation. Cosmochemistry - abundance of elements, isotope ratios in the Solar System (meteorites) Molecular Astrophysics - interstellar atoms, molecules, and interaction with radiation. Solar Spectra - spectral nature of light and resulted in the first spectroscope. ○ First proponents: Athanasius Kircher (1646) Jan Marek Merci (1648) Robert Boyle (1664) Francesco Maria Grimaldi (1665) Spectroscopy - the study of light from an object - the study of the interaction between matter and electromagnetic radiation originated through the study of visible light dispersed according to its wavelength using a prism - Experimental Spectroscopy allowed for the detection of an array of molecules within solar systems and the surrounding interstellar medium. - first used as an astronomical technique in 1802 with the experiments used by: ○ William Hyde Wollaston - built a spectrometer. Spectrometer - to observe the spectral lines present within solar radiation. ○ Joseph Von Fraunhofer - spectral lines were later quantified. - used to distinguish between different materials. ○ Charles Wheatstone (1835) Wheatstone’s report: sparks given off by different metals have distinct emission spectra. ○ Leon Foucault (1849) identical absorption and emission lines result from the same material at different temperatures.  ○ Johann Balmer Balmer Series: Spectral lines exhibited by samples of hydrogen followed a simple empirical relationship. ○ Johannes Rydberg (1888) Rydberg Formula: created to describe spectral lines observed for Hydrogen. Atomic Models ○ Ernest Rutherford’s Model - dense nucleus surrounded by a cloud of electrons. ○ Bohr’s Model - electrons surrounding the nucleus existed in discrete orbits, much like planets orbiting the Sun. Ground State - electron in its default orbit. Excited State - an electron must absorb energy. ○ Wave Model - an atom is a sinusoidal wave traveling in space with an oscillating electric field and perpendicular magnetic field. Amplitude (A) - height of wave’s electric vector. Wavelength (λ) - distance (nm, cm, m) from peak to peak  Electromagnetic Spectrum ○ the range of all types of electromagnetic radiation, each characterized by its wavelength or frequency ○ The complete variety of radiation is used throughout spectroscopy. ○ Different energies allow monitoring of different types of interactions with matter. Visible Light (350-780 nm) - radiation that can penetrate our atmosphere and be detected on the Earth’s surface. Photon ○ tiniest particle of light at a certain wavelength. ○ requires the release of the same energy that enabled them to become excited in the first place. Eyes - detectors which are designed to detect visible light waves or visible radiation. Examples of Spacecraft/Robotics with Spectrometers ○ Mars Exploration Rover launched in search of and analyzing rock and soils on Mars. Mini-TES (Miniature Thermal Emission Spectrometer) - rock, soil, and atmosphere. MB (Mossbauer Spectrometer) mineralogy of rocks and soils. APXS (Alpha Particle X-ray Spectrometer) analyze elements in rocks and soils. ○ Cassini-Huygens launched to gather information on Titan VIMS (Visual and Infrared Mapping Structure) data about the surface, rings, and atmosphere of Titan and Saturn. CIRS (Composite Infrared Spectrometer) heat and information on the object’s composition.  Spectra ○ pertains to the range of electromagnetic energy separated by wavelength. Types of Spectra ○ Continuous Spectrum also called thermal or blackbody spectrum energy of all wavelengths The emitted spectra of stars, planets, and moons are all continuous and it depends on temperature The rainbow is a continuous spectrum if it does not have any gaps In a prism, the dispersion of white light into a rainbow of colors happens because light of different wavelengths/colors refracts by different amounts inside the prism. Longer wavelength - cooler objects (approaching red) Shorter wavelength - hotter objects (approaching violet) ○ Discrete Spectrum energy at a particular wavelength Emission (bright lines) - radiative energy is released by the material. Absorption (dark lines) - energy from the radiative source is absorbed by the material.  Doppler Effect (Doppler Shift) ○ change in frequency of a wave in relation to an observer who is moving relative to the wave source. ○ Christian Doppler (1842) ○ blue light has a higher frequency than red light. ○ Recessional Velocity Hubble’s Law ○ Hubble Constant = 70 km/s/Mpc ○ The Age of the Universe (time elapsed since the Big Bang) according to Physical Cosmology is around 13.8 billion years as of 2015.  Atoms ○ smallest constituent unit of a chemical element. Molecules ○ group of two or more atoms that are held together due to chemical bonds. Compounds ○ substances that are formed when two or more elements mix chemically. Dispersion of Chemical Elements ○ Big Bang Nucleosynthesis: hydrogen, helium, etc. ○ Stellar Nucleosynthesis - carbon and oxygen ○ Supernova Nucleosynthesis - isotope of iron ○ Formation of elements inside the star, supernovae scatter them. The intense heat of supernovae are spread out to the interstellar medium. Cosmic Abundance of Elements - the comparison of the ratio of the elements in occurrence to a given environment. ○ Baryonic Matter: 4.6% of the Universe ○ Dark Matter: 27% of the Universe ○ Dark Energy: 68% of the Universe ○ Ten most common elements in the Milky Way Galaxy: Hydrogen, Helium, Oxygen, Carbon, Neon, Iron, Nitrogen, Silicon, Magnesium, Sulfur Intergalactic Space ○ the actual space between galaxies.  Interplanetary Space ○ defined by the solar wind, or the stream of energized particles from the Sun, that reaches to a point in the solar system which creates a heliosphere. Interstellar Space/Interstellar Medium ○ the material which fills the space between the stars; includes gas in ionic, atomic, and molecular form, dust and cosmic rays ○ has 10^37 tons of matter in our galaxy alone ○ Supernovae create shock waves through the interstellar medium, compressing, heating up to million degrees. No supernovae, no new stars. Interstellar Gas ○ 99% of the ISM is composed of interstellar gas ○ Composed of 75% hydrogen (molecular & atomic), 25% helium ○ It is extremely dilute with an average density of about 1 atom per cubic centimeter. ○ Cold clouds of neutral or molecular hydrogen that serve as the birthplace of new stars if they become gravitationally unstable and collapse. ○ Emits radio waves. ○ Ionized hydrogen is produced when large amounts of ultraviolet radiation are released by the newly formed stars. ○ Emission Nebulae can be seen when visible light is emitted when electrons recombine with the ionized hydrogen. ○ Ionized Hydrogen (H II) regions visible near red hot stars UV ionizes hydrogen only a small portion of the IG. ○ Neutral Hydrogen Clouds do not emit visible light can be seen when light from a star passes through the cloud. 21-cm Radiation: Neutral hydrogen can be collisionally excited so that its proton and electron have aligned spins. When the electron spontaneously flips, the atom loses a 21-cm photon. ○ Ultra-hot Interstellar Gas temperature of millions of degrees materials from supernova explosion.  ○ Molecular Clouds (dark area) a dense region where complex molecules can be formed such as ammonia, benzene, acetylene, etc. Interstellar Dust ○ made of very different substances. ○ dust does not emit visible light but it does block light, and reflects light from nearby stars ○ cooler temperatures means it emits infrared light. ○ shorter wavelengths (blue) are more easily scattered than longer wavelengths (red). ○ When there is more dust, fainter and redder the interstellar dust appears. ○ Dark Nebulae When light from other stars passes through dust, few things can happen. Light will be completely blocked, leading to dark areas. ○ Extinction caused by the light being scattered off of the dust particles out of our line of sight, preventing the light from reaching us or light passing through a dust cloud may not be completely blocked, although all wavelengths of light passing through will be dimmed somewhat. ○ Reflection Nebula region of dusty gas surrounding a star where the dust reflects the starlight, making it visible to us. Molecular Synthesis in Interstellar Gas Clouds ○ A Molecular Cloud is an interstellar cloud of gas and dust which molecules can form. The most common is the Hydrogen (H2) ○ Also referred to as a Solar Nebula ○ Molecular clouds are cold, dark, and giant. ○ Stellar Nurseries are giant condensations of dust and molecular gas. All stars are born in molecular clouds including our Sun. ○ Molecular Hydrogen has no emission line spectrum and is invisible. ○ Over 118 molecules have been detected in molecular clouds, some as complicated as the amino acid glycine. Amino acids - building blocks of proteins. Proteins - building blocks of DNA. DNA - building block of life. Timeline of The Early Universe Big Bang Theory – originally conceptualized by German Priest George Lemaitre. 3-20 minutes (Nucleosynthesis) ○ Temperature drops for another billion degrees - making it possible for the atomic nuclei to be formed ○ Nuclear fusion in the space forms Hydrogen, Helium, and Lithium ○ 17 minutes after, nuclear fusion in the space is no longer possible 3 minutes - 240,000 years (Photon Epoch) ○ Soup of atomic nuclei ○ Photons could travel much because the space was not "transparent" ○ At 100,000 years, most of the energies are X-rays, radio waves, and ultraviolet 240,000 - 380,000 years (Recombination and Decoupling) ○ Recombination forms neutral atoms ○ The space is now "transparent" ○ Cosmic Microwave Background radiation relic ○ 75% Hydrogen, 25% Helium, and Lithium traces 300-500 million years (Star formation) ○ First generation stars were born; more massive 100x, hotter 20x, luminous 10x, ultraviolet blue ○ Hypernova Proton-Proton Chain: Converts hydrogen into helium in stars similar to or smaller than the Sun, driving their energy production. CNO Cycle: Converts hydrogen into helium using carbon, nitrogen, and oxygen as catalysts, dominant in more massive stars. Triple Alpha Process: Converts helium into carbon in the cores of red giants, requiring extremely high temperatures and densities. Stoichiometry A section of chemistry that involves using relationships between reactants and products in a chemical reaction to determine desired quantitative data. reactants are displayed on the left side, products are on the right. Gas Phase Reactions are the simplest chemical reactions that occur in the gas phase in a single step. Electronegativity ○ measure of the tendency of an atom to attract a bonding pair of electrons.  Electron Affinity is defined as the change in energy (kJ/mole) of a neutral atom when an electron is added to the atom to form a negative ion. Ionization refers to the splitting up of molecules of a substance into positive and negative ions when the substance is dissolved. Types of Chemical Reactions ○ Synthesis a compound is made from simpler materials ○ Decomposition a compound broken down into simpler compounds or all the way to the elements that make it up ○ Combustion compounds containing carbon and hydrogen combine with oxygen gas to produce carbon dioxide and water ○ Single Replacement one element that starts out itself by replacing another element in a compound, kicking it out ○ Double Replacement positive and negative ions in two compounds switch places  Types of Chemical Bonds ○ Ionic Bond Metal atom loses electron(s) to nonmetal atom ○ Covalent Bond Two nonmetal atoms share electrons ○ Hydrogen Bond Hydrogen attracts an electronegative atom electrostatically ○ Metallic Bond Positive metal ions attract conducting electrons Nomenclature Organic Compounds Organic Compounds is a large class of chemical compounds in which one or more atoms of carbon are covalently linked to atoms of other elements, most commonly hydrogen, oxygen, or nitrogen. Carbon has 4 valence electrons in the outer shell, likes to constantly bond with other molecules to each 4 electrons, bonds in all directions. Aliphatic compounds are arranged in straight continuous carbon chains, branched structures, and have high carbon-to-hydrogen ratio. Aromatic compounds are arranged in ring structures with delocalized pi electrons, and have low carbon-to-hydrogen ratio. Isomers are compounds that have the same chemical formula but different structures.  Functional groups determine the characteristics and properties of the organic compound (each act differently). It contains CHO in some way (and sometimes NP). All are hydrophilic (love/are attracted to water and dissolve in water easily) R-groups are functional groups that are attached to a Carbon-Hydrogen chain combination. Because some of the attached groups can be so large, we shorthand them as "R" group Carbonyl Group is a chemically organic functional group composed of a carbon atom double-bonded to an oxygen atom --> [C=O] ○ The simplest carbonyl groups are aldehydes and ketones usually attached to another carbon compound. ○ R represents a hydrogen atom or carbon/hydrogen chain, CO represents the carbonyl, and H represents the hydrogen attached to the carbonyl chain  Carbonyl Hybridization Aromatics ○ Criteria for Aromaticity: 1. Cyclic a ring of atoms (Benzene) 2. Planar - all atoms in the molecules lie in the same plane 3. Conjugated - p orbitals at every atom in the ring 4. Hückel's Rule (4n+2 π electrons) Odd electron pairs = aromatics ○ It is antiaromatic if all of this is correct except it has 4n electrons, Any deviation from these criteria makes it non- aromatic. IR Spectroscopy Graphs Carboxylic Acid (O-H) ○ Occurs around 3000-2500 cm^-1 ○ We can determine a carboxylic acid by looking for a strong, broad peak. A carboxylic acid has two components, there must also be a C = O in zone 4 to have a carboxylic acid Alcohols (O-H) ○ Occur around 3650-3200 cm^-1 ○ Alcohols have a very distinct strong and broad shape. When we see this sort of elongated "U" shape around this region, we know there is an alcohol group. Aldehydes (C-H) ○ One peak around 2900 cm^-1 and another around 2700 cm^-1 ○ Aldehydes are of medium length and have two peaks. The first peak is often distorted by sp³ carbons and may not be seen. Look for the second peak around 2700 cm^-1.  Ketones (C=O) ○ Around 1750-1705 cm^-1 ○ Ketones will have a strong peak. Look for the number of photons in the low 1700s. This may not always be the case, however. Remember to look for signs of conjugation. Esters (C=O) ○ Around 1750-1735 cm^-1 ○ Esters will have a strong peak. Numbers around the 1740s range are typical. This may not always be the case, however. Remember to look for signs of conjugation.