Topics Class-Dr. Brian Barry
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| if you think you synthesized "something" new or interesting or know a compound, how do you convince someone else that you know what you made? | look in literature; use an array of necessary/appropriate analysis methods to make your case
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| chemical analysis is key for quantitative information on | atomic compositions and chemical reactivity
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| chemical analysis of metals | atomic absorption/emission spectroscopy (AA/AE)
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| chemical analysis of gases/organic compounds | oxygen-based combustion analysis (CHN converted to CO2, H2O, N2 gases)
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| chemical analysis of ions | classical ion-selective detection (ex. flame tests)
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| most chemical analysis methods require the destruction of the chemical uniqueness and atomic connectivity of your | inorganic/organic materials (aka sample)
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| what types of "physical" analyses are applicable to inorganic chemistry? | that depends strongly on what type of inorganic structure/material you are studying
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| which is easier to study molecules or extended materials | molecules because they are easier to dissolve
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| when the nuclear spin quantum number (I) value = 0 | the nuclei is unaffected by a magnetic field (ex. 12C)
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| as the atom size increases the Larmor frequency | decreases because it takes longer to turn over
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| T1 is the relaxation time also described as | the time required for the nuclei to come back up after being knocked down; different rate for each nuclei
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| 1 T is how many MHz | 42.6 MHz
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| a magnetic field of 1T induces a Larmor frequency of how much in a proton | 42.6 MHz
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| for a given nucleus with a nuclear spin, I there are how many degenerate states possible | 2I +1
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| NMR is a super conductor containing electrons rotating | counterclockwise
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| in NMR a constant magnetic field is generated from electricity, the stronger the field is it easier or harder to flip an electron | harder
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| free induction decay | wavelength released from a charged electron as it flips back to it's relaxed mode
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| Fourier transforms changes the RF signal (intensity vs. time) to | x-axis of ppm
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| how does ppm relate to frequency | range is roughly 15-0 ppm for 1H NMR but different standards can be chosen (scaling factor)
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| absorption of energy by nucleus with a nuclear magnetic moment is called | resonance (spin changes relative to applied magnetic field). The change in energy is dependent on magnetic field strength
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| a brief (microsecond) RF pulse causes transitions from ground nuclear spin state to | excited nuclear spin state transition (M (I)= +1/2, -1/2)
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| sensitive RF detector coil records the | time spectrum that is converted to frequency domain by FT
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| most modern magnets are not perfectly stable, so they need to maintain a lock on a specific nucleus and then adjust the | field slightly to keep the resonance energy of this nucleus unchanged
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| since energy increases as the magnetic field increases recording absorption processes on different instruments | will lead to different frequency values
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| since only ppm differences exist with different resonance frequencies, it is crucial that all nuclei see the same external magnetic field; how do we make it homogeneous | a. spin a dissolved solution sample very fast (~20 Hz) b. fine tune imperfections in applied magnetic field and glass tube by electronic shimming
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| why do peaks appear at different relative resonance energies/frequencies? | define Heff=net magnetic field implying on the nucleus of interest
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| chemical shift is dimensionless but is generally reported in ppm to | make it the same on different NMR
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| EPR | sample is held in a very strong magnetic field while electromagnetic radiation is applied monochromatically
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| Magnetic susceptibility | is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field (k)
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| diamagnetism | materials create an induced magnetic field in a direction opposite to an externally applied magnetic field, and are repelled by the applied magnetic field.
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| paramagnetism | a form of magnetism whereby certain materials are attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field.
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| ferromagnetism | the basic mechanism by which certain materials (such as iron) form permanent magnets, or are attracted to magnets
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| units for u (mu) | magnetic moment; A(m^2) or J (T^-1)
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| CHNS beings by wrapping a sample in | aluminum foil
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| CHNS sample is dropped into a chamber with | 5% He (carrier gas) and 95% O2 (combustion) and the sample hits a heated filament
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| CHNS the sample is combusted and the volitle components are carried away by | He
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| In CHNS we need to decrease oxygen to the point that it is | out of the gas stream and we do this with high surface area copper catalyst
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| how to we get rid of the oxygen in CHNS | high surface area copper catalyst; CuO forms on surface and you are left with He, CO2, H2O, N2, and SO2 which enter the GC
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| what components of CHNS enter the GC | He, CO2, H2O, N2, and SO2
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| using standards in GC we can relate peak intensity to the | concentration/mass of the sample
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| what are somethings that can go wrong in CHNS? | not all of it combusts; metal carbides form
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| if carbides form in CHNS the GC readout will have a low levels of | carbon because not all of it reacts
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| how do you counteract the formation of carbides in CHNS | add WO3 and/or Ta2O5 catalysts which catalyze the combustion (make the reaction happen so fast that the metal carbides can't form)
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| what does CHNS detect? | carbon, hydrogen, nitrogen, and sulfur
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| solid state chemistry deals with | extended materials held together by ionic or covalent bonds
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| in extended materials where are the electrons | shared over the whole material; not localized
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| extended materials have a higher boiling point and a lower volatility than | molecules
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| solid-state chemistry is the study of | well-ordered crystals, often flawless on near (mm) length scale; syntheses target new compositions with often complex structures and new physical properties
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| materials chemistry is the | synthesis & study of extended solids (ex. highly disordered structures, low dimensional polymers, solids with nanoscale dimensions); often manipulate known solids to achieve new & useful physical or chemical properties
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| 4 general characteristics of molecules and molecular solids | discrete units (usually under 10 covalently bonded atoms), valence e-s localized bonds or MOs, synthesis often at low temperatures, may be soluble
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| 3 general characteristics of solid-state structure | large number of interconnected atoms or ions held together by covalent and/or ionic forces, bonding by valence e-s in orbitals that are dispersed in bands, synthesis at high temps
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| the ceramic method is also known as | "heat and beat" or "shake and bake"; mix and heat to react; directly react the components at high temperatures
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| Thin film growth | chemical vapor deposition (CVD); A(g) + B(g) = C(s)film + byproduct gases
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| solution nanoparticle growth (<100 nm particle sizes) | particle growth (nucleation and precipitation) from eated solution of reactive precursors-AB compounds may require a subsequent high-temperature annealing (or use very hot solutions) step to complete reaction and form crystalline product
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| we need a reaction vessel that removes air/O2 from reaction how do we solve this | load both reagents into a sealed, evacuated glass ampoule
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| remember PV= | nRT calculation for gaseous reagents, intermediates, and byproducts
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| stable solid reagents (e.g. Ti Metal) require substantial external energy to react; so we need to heat the elements in a furnace and the thermal limits of the | reactor need to be considered
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| ampoules for heating air-sensitive reactions are made from | pyrex (use up to ~550 degrees C in a vacuum); silica (pure SiO2 up to ~1200 degrees C)
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| SiO2 will not crack upon rapid cooling (drop in water) but Pyrex | will (thermally expansion coefficient is large)
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| what are the experimentally determined "best" reaction conditions for TI and S8 | mix powders in a sealed evacuated Pyrex ampoule; heat at 400 degrees C, intermittently cool, remove partially reacted powder, grind it to expose reactive Ti surfaces, and reheat in an ampoule; heat in a silica vessel at high Ts for a week
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| problems with glass ampoules | reagent attack on glass walls; ampoule explosions due to gas pressure release
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| how do you solve reagents sticking to the glass ampoules walls | add a second protective liner to the sealed glass ampoule (dual containment)
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| how do you solve an ampoule explosion due to gas pressure release | use a glass tube with flowing inert gas
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| resistively heated components (controlled current/voltage through metal wire) | heating tape or mantles; lab tube or box furnaces; furnaces with special heating elements; resistive heating (inert conditions) of graphite or a metal foil
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| exotic heating methods | induction heating; electric arc/discharge; high wattage halogen bulb or infrared bulb heaters
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| grinding helps maximize the | surface area of compounds
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| two solids can "diffuse" into each other but they would take | an incredibly long time to do this by themselves
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| NMR is a resonance technique involving absorption of what type of frequency energy | radio frequency energy
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| the magnetic environment of a nucleus affects its resonance frequency and allows what type of information to be deduced (hint NMR) | structural information
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| to be NMR active the nuclei must poses a nuclear spin (nuclear angular momentum) (I) that does not equal | 0
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| In the absence of an applied magnetic field, the different nuclear spin states of a nucleus are | degenerate
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| Are the nuclear spin states of a nucleus degenerate in the presence of an applied magnetic field? | no
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| With nearly 100% abundance the 1H NMR spectrum is contributed by how many hydrogen atoms in the sample | virtually every one
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| Which nuclei are suitable for NMR spectroscopic studies? | ones with an I value that doesn't equal 0; helpful if it exists in high abundance; helpful if the T1 (relaxation time) is relatively short
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| possession of a quadrupole moment leads to short values of T1 but tend to cause the peak to | broaden
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| a particular nucleus absorbs characteristic radiofrequencies (it resonances at the frequency). If an NMR spectrometer is tuned to a particular resonance frequency | only a selected NMR active nucleus is observed
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| ex. only 1H nuclei are observed if a 400 MHz spectrometer is tuned to 400 MHz, but if the same spectrometer is returned to 162 MHz, only | 31P nuclei are observed (just like tuning a radio and receiving one station at a time)
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| Nonequivalent nuclei of the same element resonate at different frequencies and therefore have different | chemical shifts
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| although chemical shift does not technically have units it is recorded in ppm so as to show that | the value was multiplied by 10^6; it was multiplied because the values were so small
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| different reference compounds can be used to place 0 ppm; one example is | TMS; CD3Cl
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| the closer a ppm value is to 0, is the frequency higher or lower | lower
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| deuterated solves are used so that | the 1H NMR spectra are not swamped with unnecessary hydrogen signals
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| integration of a peak is proportional to | the number of nuclei giving rise to the signal (ex. 1:3)
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| Coupling results from one hydrogen's magnetic field being associated with | another hydrogen's magnetic field
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| coupling constant J is measured in Hz and describes | the distance between two coupled peaks
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| proton-decoupled is represented as {1H}, what are decoupled experiments | NMR experiments where certain nuclei are "hidden" on the radiofrequency
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| What is the Larmor frequency of a 31P in 3T magnetic field? (note in 100 MHz NMR 31P=40.5 MHz) | 51.8 MHz
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