Rotating crystal technique and single-crystal diffraction MCQs With Answer

Introduction: Understanding the rotating crystal technique and single-crystal diffraction is essential for B. Pharm students studying crystal structure determination and X-ray crystallography. The rotating crystal method rotates a single crystal through the X-ray beam to record diffraction spots, revealing lattice planes, unit cell dimensions, and symmetry. Mastery of concepts like Bragg’s law, Miller indices, reciprocal lattice, intensity measurement, and data collection strategies helps in solving molecular structures, polymorphism, and solid-state characterization of drugs. Practical knowledge of instrumentation, rotation geometry, and data reduction improves interpretation of electron density maps and validation of crystal models. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary purpose of the rotating crystal technique in single-crystal diffraction?

• To measure absorption coefficients of the crystal
• To obtain multiple diffraction spots by changing crystal orientation
• To heat the crystal evenly during data collection
• To align the crystal with magnetic fields

Correct Answer: To obtain multiple diffraction spots by changing crystal orientation

Q2. Which fundamental equation relates interplanar spacing, wavelength, and diffraction angle in X-ray crystallography?

• Planck’s equation
• Bragg’s law
• Schrödinger equation
• Beer–Lambert law

Correct Answer: Bragg’s law

Q3. In the rotating crystal method, what geometric construction describes allowed reciprocal lattice points that satisfy diffraction conditions?

• Miller index plot
• Ewald sphere
• Laue pattern
• Bravais lattice

Correct Answer: Ewald sphere

Q4. Which instrument component defines the rotation axis and angular precision in rotating crystal experiments?

• Monochromator
• Goniometer
• Beamstop
• Detector readout

Correct Answer: Goniometer

Q5. What are Miller indices used to designate?

• Intensity of diffraction spots
• Orientation of lattice planes
• Atomic coordinates in the unit cell
• Temperature factors of atoms

Correct Answer: Orientation of lattice planes

Q6. Which of the following best describes the reciprocal lattice?

• A direct-space representation of atomic positions
• A Fourier transform space where diffraction spots correspond to lattice points
• A grid used for indexing unit cell parameters in real space
• A map of thermal motion of atoms

Correct Answer: A Fourier transform space where diffraction spots correspond to lattice points

Q7. In the rotating crystal technique, why is it beneficial to rotate the crystal through a range of angles rather than a single angle?

• To reduce thermal vibrations
• To sample more reciprocal lattice points and improve completeness
• To change the wavelength of the X-rays
• To dissolve crystal defects

Correct Answer: To sample more reciprocal lattice points and improve completeness

Q8. What is “mosaicity” of a crystal as relevant to single-crystal diffraction?

• The number of atoms per unit cell
• The spread of orientation of crystalline domains within a sample
• The absorption coefficient variation across the crystal
• The crystal’s color under polarized light

Correct Answer: The spread of orientation of crystalline domains within a sample

Q9. Which detector type is commonly used for modern single-crystal diffraction to record spot intensities rapidly?

• Photographic film
• Point counter tube
• Area detector (CCD or CMOS)
• Geiger-Müller counter

Correct Answer: Area detector (CCD or CMOS)

Q10. During data reduction, what is the purpose of an absorption correction?

• To remove background cosmic radiation
• To correct intensity variations caused by X-ray absorption within the crystal
• To normalize detector pixel sensitivity only
• To convert Miller indices to real-space distances

Correct Answer: To correct intensity variations caused by X-ray absorption within the crystal

Q11. What is a systematic absence in diffraction data and what does it indicate?

• Missing data due to detector failure; indicates equipment fault
• Absence of reflections dictated by lattice centering or glide planes; indicates symmetry elements
• Spots that are too weak; indicates poor crystal quality
• Reflections blocked by the beamstop; indicates incomplete data

Correct Answer: Absence of reflections dictated by lattice centering or glide planes; indicates symmetry elements

Q12. How does increasing the oscillation angle per frame affect rotating crystal data collection?

• Decreases overlap of reflections but may reduce resolution
• Increases background noise without benefits
• Reduces the number of reflections recorded overall
• Eliminates need for absorption correction

Correct Answer: Decreases overlap of reflections but may reduce resolution

Q13. What is the role of the beamstop in single-crystal diffraction experiments?

• To focus the X-ray beam onto the crystal
• To block the intense primary beam from saturating the detector
• To cool the crystal during data collection
• To polarize the X-ray beam

Correct Answer: To block the intense primary beam from saturating the detector

Q14. Which quantity is commonly used to express the internal consistency of measured intensities during scaling?

• Multiplicity
• Rmerge (or Rsym)
• Unit cell volume
• Space-group number

Correct Answer: Rmerge (or Rsym)

Q15. Why is redundancy (multiple observations of same reflection) desirable in single-crystal diffraction?

• It decreases unit cell dimensions
• It improves intensity accuracy and enables better error estimates
• It increases mosaic spread artificially
• It eliminates the need for symmetry determination

Correct Answer: It improves intensity accuracy and enables better error estimates

Q16. What information does the resolution (d-spacing) of a dataset provide?

• The maximum unit cell volume measurable
• The minimum distance between resolvable features in the electron density map
• The amount of absorption correction required
• The number of symmetry operations in the space group

Correct Answer: The minimum distance between resolvable features in the electron density map

Q17. Which method contrasts with rotating crystal technique by using stationary crystal and multiple wavelengths simultaneously?

• Precession method
• Laue method
• Powder diffraction
• Oscillation method

Correct Answer: Laue method

Q18. What is the significance of Friedel pairs in single-crystal diffraction?

• They are reflections with identical intensities always
• Differences between them enable detection of anomalous dispersion and absolute structure
• They indicate an absence of symmetry in the crystal
• They are reflections recorded only with neutrons

Correct Answer: Differences between them enable detection of anomalous dispersion and absolute structure

Q19. Which parameter describes thermal motion of atoms in a refined crystal structure?

• B-factor (temperature factor)
• Multiplicity
• Rmerge
• Crystal mosaicity

Correct Answer: B-factor (temperature factor)

Q20. What is meant by “indexing” in single-crystal diffraction?

• Assigning Miller indices (hkl) to measured diffraction spots and determining unit cell metrics
• Calculating absorption coefficients
• Adjusting detector gain settings
• Measuring the mosaic spread

Correct Answer: Assigning Miller indices (hkl) to measured diffraction spots and determining unit cell metrics

Q21. In rotation photography with film or area detectors, what causes spot elongation when the crystal rotates?

• Detector pixel size only
• Finite rotation range during exposure and mosaic spread
• Excessive cooling of the crystal
• Use of monochromatic radiation exclusively

Correct Answer: Finite rotation range during exposure and mosaic spread

Q22. Which symmetry element can be inferred directly from systematic absences in reflection intensities?

• Atomic thermal vibration
• Glide plane or screw axis
• Occupancy disorder
• Presence of heavy atoms only

Correct Answer: Glide plane or screw axis

Q23. During collection, why is cryocooling often used for single crystals in diffraction experiments?

• To increase mosaicity for easier indexing
• To reduce radiation damage and lower thermal motion
• To change the space group
• To make the crystal more absorbent

Correct Answer: To reduce radiation damage and lower thermal motion

Q24. What is the typical next computational step after indexing and integration of reflections?

• Direct measurement of bond lengths
• Scaling and merging of intensities
• Applying thermal vibrations manually
• Choosing the beamstop position

Correct Answer: Scaling and merging of intensities

Q25. How does anomalous dispersion assist in single-crystal structure determination?

• It reduces detector noise
• It provides phase information and helps locate specific atoms (e.g., heavy or anomalous scatterers)
• It corrects unit cell parameters automatically
• It increases unit cell symmetry

Correct Answer: It provides phase information and helps locate specific atoms (e.g., heavy or anomalous scatterers)

Q26. What is extinction in the context of X-ray crystallography?

• A loss of crystal after irradiation
• Reduction in observed intensity due to multiple scattering within a perfect crystal
• Complete absence of diffraction due to amorphous sample
• Destruction of detector pixels over time

Correct Answer: Reduction in observed intensity due to multiple scattering within a perfect crystal

Q27. Which of the following best describes “precession photography” often used historically for single crystals?

• A method to cool crystals to very low temperatures
• A geometry that keeps a reciprocal lattice plane in focus by precessing the crystal and beam
• A modern area-detector readout algorithm
• A technique to measure absorption coefficients

Correct Answer: A geometry that keeps a reciprocal lattice plane in focus by precessing the crystal and beam

Q28. What is the role of the monochromator in single-crystal X-ray experiments?

• To vary the crystal temperature
• To select a narrow wavelength band from the X-ray source
• To physically rotate the crystal
• To amplify detector signals

Correct Answer: To select a narrow wavelength band from the X-ray source

Q29. Which parameter indicates how well the calculated model fits the observed diffraction data after refinement?

• Mosaic spread
• R-factor (Rwork and Rfree)
• Beam intensity
• Unit cell angles only

Correct Answer: R-factor (Rwork and Rfree)

Q30. In the context of pharmaceutical crystals, why is single-crystal diffraction important for drug development?

• It determines the solubility directly without experiments
• It provides precise atomic-level structures to understand polymorphism, stability, and molecular interactions
• It replaces all required biological assays
• It is only used for color analysis of crystalline powders

Correct Answer: It provides precise atomic-level structures to understand polymorphism, stability, and molecular interactions

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